Pianos do go out of tune with the seasonal changes in the weather and how much the piano is played. It is best to tune a piano on a regular basis in order to keep it in the best condition possible. Most people should have their piano tuned at least twice a year if they use it on a regular basis. Many artists have their pianos tuned even more frequently. Pianos used for concert performances are almost always tuned and checked for trouble before every performance.
Many times when the piano is tuned, the piano tuner will attempt to look for any signs of trouble. If there is any meaningful signs of trouble that could affect the piano, the piano tuner will try to make the customer know so that proper action can be taken to correct the trouble if necessary. If you haven't had your piano tuned for quite some time, there is a chance that it will need more than one tuning in order to bring it to the correct pitch (A-440). Be aware that there is the chance that a string can break in the process of tuning a piano. If only one or just a small number of strings should break, they can be replaced. However, if the strings are in poor condition, it is doubtful that the piano can be tuned to the correct pitch. If it is desired to have the piano at the correct pitch, major repairs will be necessary by a specialist.
Action regulation consists of adjusting the action so that it will perform properly. This also in many instances includes tone regulation where the felt hammers are either softened by poking them with needles or hardened by using a special lacquer. Also when the hammers develop grooves in them that are too deep, they are carefully filed to restore their shape. However, this cannot be done forever as in time it reaches the point where the is not enough felt remaining on the hammers to file them properly.
Restringing a piano consists of replacing all of the strings in the piano with brand new strings. Though in rare instances this is done in a customer's home, most of the time, the piano is brought to a shop where this major repair is performed.  When the strings are replaced, the tuning pins are also replaced.
Rebuilding involves repair or replacement of the major components of the piano. When a piano is rebuilt, not only are the strings replaced, but many times major repairs are performed on the soundbaord and bridges, action (mechanism that controls the hammers) keys, hammers (replaced if necessary), pinblock replacement (part that holds the tuning pins) if necessary and refinishing if necessary. Soundboards are replaced in the event that the existing soundboard is in too poor of condition to be repairable.
Refinishing a piano is probably the most labor intensive part of rebuilding (restoring) a piano since there are so many parts of a piano that need to be stripped of the old finish and then refinished. Refinishing a piano will often cost as much if not more than rebuilding a piano with a new soundboard, pinblock, action (hammers included). Try to avoid refinishing except in the case where the existing finish on a piano is so far gone where nothing will help to restore the existing finish to an acceptable condition. If the existing finish is simply dull from age, polishing can often rejuvenate it. Rejuvenating an existing finish will cost a fraction of that of stripping and replacing the finish.
Anyone who plans to buy a piano new or used should consult with an experienced piano technician that has a good record of integrity in their local area. The Piano Buyer Guide by Larry Fine can be a good guide for those who are looking to purchase a new or used piano.

Nearly all pianos that have been manufactured since about 1880 or so fall into 2 basic categories. Verticals (spinets, consoles, studio uprights, full uprights) and grands (all sizes).
A vertical piano is a piano where the strings and soundboard are perpendicular (vertical) to the floor. A grand piano is generally a harp shaped piano where the strings or parallel (horizontal) to the floor. Vertical pianos fall into 4 basic categories. A spinet piano is the smallest of the vertical pianos at 36" - 39" in height. A console piano generally ranges from 40" - 43" in height. A studio upright ranges from 44" - 47" in height. A full upright (sometimes called professional upright) is 48" or taller.

Grand pianos also fall into different size categories. A baby grand is a grand no more than 5' 8" in length. A living room grand is generally 5' 9" to around 6' 0" in length. A Drawing Room grand ranges from 6' 0" to around 6' 8" in length. A parlor (music room) grand is often around 7' 0" in length. A parlor concert grand is generally 7' 6" in length. A full (orchestral) concert grand is generally around 9' 0" in length.

Spinet pianos are often not the most desirable pianos musically and technically due to their small size. In order to maintain the proper keyboard height and the position of the action in relation to the striking point on the strings, the action must be located at least partly and usually almost completely below the level of the keyboard. This is accomplished by reducing the length of the keys by a few inches and then providing a connecting part from the key to the action. The result of this is a piano that is quite difficult to service because the action is in a confined space that makes it quite difficult to remove for servicing. Thanks to the short strings in the bass and the critical tenor area of the piano, these pianos are often difficult to impossible to tune properly.

A console piano has a direct action which sits on top of the keys and is much easier to service. However, this is a compromise as the action parts themselves have to be more compact in order the fit properly within the inside of the piano. In most instances this is generally an acceptable compromise as the feel of the keyboard in a console is much better than that of most spinets. The longer strings in the bass an tenor areas make is possible to tune these pianos more accurately.

Studio uprights differ from consoles as they have a full sized direct action and longer strings in the bass and tenor sections. The full sized direct actions make these pianos relatively easy to service compared to a console and certainly a spinet. Many of these pianos have utilitarian case styling which often makes these pianos fine for homeowners where the case styling is of little importance. Thanks to a soundboard that is larger in area than that of a typical spinet or console, these pianos have a better sound than almost any smaller piano.

A full sized upright is a good choice for those who need a good sounding piano but they do not have the space or the money for a decent grand piano. Like the studio upright, these pianos have a full sized direct action. In the largest of these pianos, there are connecting parts called stickers (abstracts) that go from the top of the key to the action. These stilt like extensions are used when the action needs to be located far above the level of the keys. They do not diminish from the feel of the keyboard. They are simply an extension of the action.

Grand pianos also vary in desirability. The smallest grands (less than 5' 0" in length) tend to come up short musically and they are often placed at the lower end of the product line by most manufacturers. Once one reaches 5' 0" and up, both the sound and the quality improves. I would suggest buying a grand closer to 5' 6" length as the sound is often noticeably better than that of a 5' 0" grand. A grand around 6' 0" is a better choice since the sound overall is better. Many professional performers will try to obtain a grand around 7' 0" in length as they can often perform better than that of a 6' 0" grand. Few people can either afford or fit a 9' 0" grand into their homes. Also a 9' 0" grand would overwhelm most rooms in a typical home due to their tonal power.

If you plan to buy a used piano I'm going to provide a rough guideline here upon what to expect for the price you wish to pay.

If your budget is limited to $500 maximum, you are not going to have much luck finding a usable piano.

If you can budget $1,000 +/-, you might be able to obtain something usable, but musically uninspiring.

For around $1,500 you'll probably be able to buy a usable piano from a dealer that is still in decent enough original condition or the piano has been reconditioned. However, don't expect to get a great piano.

$2,000 will probably get you an older piano of a better brand in decent enough shape, but a newer piano in the same price category will probably not be the greatest of pianos.

$3,000 or so will likely get you a decent enough used vertical piano in good condition.

$4,000 or so will likely get you a used vertical piano for a more advanced student.

$5,000 or so will get you a more recently made vertical (1990s) that would be suitable for an advanced student.

If you planning to purchase a used grand piano, plan upon spending $7,500 at the minimum on a used grand piano that is of adequate enough quality.

$10,000 will get you a very nice used grand piano.

Plan upon spending at least $15,000 to even $20,000 at the minimum for an older used Steinway that is still in good enough condition. Keep in mind that such a piano will eventually need to be rebuilt in all likelihood. A rebuilt Steinway grand will probably cost $30,000 at the least and do expect to spend $40,000 or more for such a piano.

In general, expect to pay more for a larger grand piano than a smaller one of the same quality and condition.

Don't be tempted to purchase one of those small, inexpensive grands or an older small grand that is being sold for relatively little money. They do not perform well as a rule. Spend the money on a decent vertical instead.

When purchasing a used piano from a private party, do have a technician evaluate the piano for any possible problems. As pianos age, they do deteriorate. While a properly cared piano might not have any significant problems, and older piano that has not been maintained properly or one that was maintained properly and received heavy usage or simply a very old piano can have significant issues. Hammers can wear out from extended usage. Pinblocks can fail due to age result in some tuning pins that are too loose to hold the affected strings in tune. Bridges can split which can often result in poor tone on the affected notes. Strings can become rusty which can often result in poor tone and breakage. Actions can wear out resulting in notes that do not play correctly or do not play at all. On very rare occasions the iron plate (large brass colored metal harp shaped piece) that holds the strings in place can crack rendering the piano useless. While new techniques have been developed that can successfully repair a cracked plate, it is best not to obtain a piano with a cracked plate since the repair can be difficult and it may not always work. Sometimes, the pinblock on a vertical piano and even the rest of the top back of the piano can start to pull away from the back posts. Repairs are possible, but may not be worthwhile in a piano that was entry level to begin with or on a piano that is generally in poor condition otherwise. On rare occasions, the rim of a grand piano can start to come apart or the support beams can start to come apart. Again repairs are possible but should not be considered for an entry level grand piano. Soundboards do crack, but it often does not mean death to the piano. If the cracks are small, they can generally be left alone. However, if the soundboard is pulling away from the ribs, or vice versa, repairs are likely to be needed and they can be costly. If the soundboard is too far gone, replacement will be required and is only worthwhile on the more expensive pianos.

If you plan to buy a used piano from a dealer, make sure that it is being sold with a warranty. Any used piano in good enough condition should be sold with at least a one year warranty to play it safe. However, not all used pianos will be sold with a warranty. Beware of any used piano sold by a dealer in as is condition. It could mean that the piano will need a significant amount of repair or worse yet, may have so many issues that the piano is completely useless as a musical instrument and might not even be worthwhile repairing.

Once you have purchased your used piano, do plan upon moving it. If you have purchased the piano from a dealer, the cost of the move should generally be included. If you have purchased your used piano from a private party, plan to hire professional movers to move the piano. Do not attempt to move the piano yourself even if you have helpers. Pianos are heavy and moving them is very unforgiving. Make sure that the piano mover that you choose is bonded and insured in case anything should happen.
Pianos in institutions are often subjected to harsh conditions. There is often little or no climate control. The pianos themsleves can be subjected to heavy usage. It is extremely important that anyone who is in charge of pianos within an institution makes sure that enough money is budgeted for the maintenance of the pianos.