Almost all piano manufacturers suggest tuning a new piano four times in the first year and then twice a year thereafter.  At a minimum, a piano should be tuned at least once a year.

Playing a piano will certainly cause it to go out of tune.  However, changes in temperature and humidity will also cause the piano to go out of tune, even when it's not played much.  The amount of use a piano gets does affect its tuning, especially a pianist that plays extraordinarily hard.

Letting a piano get very far out of tune isn't good for it, but the harm is not typically irreparable.  A piano adjusts to the tension of its strings (typically 38,000 lbs of pressure).  When a piano is allowed to go out of tune, the string tension is reduced and the piano settles to the new lower tension.  When the strings are pulled up to proper pitch again, the piano typically requires days or weeks to settle again.  As it adjusts the string tension is changed slightly and the piano needs tuning again.  As a result, pianos that are neglected, even for a few years, often require two or more tunings to reach stability, while a piano that is kept reasonably in tune holds its tuning better.

There are several problems that your piano could have.  Most commonly the tuning pinblock isn't holding the tuning pins tightly enough.  The pinblock is made of several layers of wood laminate, typically hard rock maple.  Holes are drilled into it that are slightly smaller than the tuning pins that are inserted.  The friction of the pin on the wood should hold the tuning pin in place against the tension of the string (usually about 180 lbs per string).  If the pin has become loose in the tuning pin block, the pin will slip and release the pressure on the string.  Sometimes the piano technician can apply a simple remedy, but often the tuning pin block is just worn out.

Other tuning problems are caused more by the environment than something inside the piano itself.  If the piano is next to a door or window or the temperature or humidity in the room fluctuates substantially, the piano's wood and other parts will expand and contract causing the pressure on the strings to change.  The remedy is stabilizing the temperature and humidity around the piano.  Often installing a Damp-Chaser — a device to stabilize the humidity — will help.

A piano tuner typically just tunes pianos.  A piano technician can do complex repairs like action regulation, voicing, repairing broken parts, and even completely rebuilding a piano.  But most piano technicians don't object to being called a piano tuner because it's a name the public recognizes.