The Chobits anime had various ending theme songs, as well as character songs; 5 volumes of drama CDs were also released. In total, 12 CDs were released for the Chobits anime.
Singles are most often the opening, ending or insert songs for an anime. Usually 2 to 5 tracks are on one CD.
A soundtrack (Also known as an OST) is a compilation of songs and background music (BGM) for a series. Songs can include theme songs and insert songs. CDs can have up to 30 songs, depending on the length of the scores.
Compilation albums may occur when a series has a number of theme songs and insert songs. The compilations may host instrumental or karaoke versions of the tracks.