Original Soundtrack (Often shortened to OST) is a collection of music in a show. It can include background music, insert songs, and sometimes opening themes and ending themes. Usually people are able to purchase a tape or CD containing a collection of these music tracks. If a series is rather long and has too many soundtracks to release on a single CD, then there will often be several OST releases. Often being labeled as OST 1, 2, etc.
Most OSTs do not contain opening themes and ending themes, this is usually due to licensing restrictions.
Usually due to time restrictions, shows will usually only use a portion of music used. A good example would be opening themes and ending themes. Most OSTs will contain the full piece of the music, giving a chance for fans to hear the entire piece. Although OSTs that only contain the 'television sized' portions of music do exist.
There is also Image Albums, which contain music that are usually composed before the show's production begins. The music is generally made when the composer is reading the script of the anime, and begins to compose music based oh his or hers mood or imagination after reading the script. Eventually the music will be aired when the scores have been revised to match the animation. Many OSTs have music from Image Albums incorporated into them.
Many OSTs are not licensed in America. But as anime continually grows more popular, so does the demand for OSTs. And imports from Japan are commonly rather expensive. This soon resulted in a market that made bootlegged versions of OSTs available to the American public.