It was originally used to distinguish between Singles - which used to have only one track per side. EP's (Extended Players) - which had up to two or three tracks per side, and LP's (Long Players) - which had entire albums worth on...
LP's, because they had so much on them though, used to be less quality (something to do with the quality of vinyl and how much info it used to be able to contain over a certain amount of grooves etc) so they were, and still are recorded on a lower volume than a twelve etc....
EP's don't really suffer with this......