What's the difference in an ep and a lp?

Album is considered a complete body of work for the recording artist; it is termed full-length or LP (Long Play) because preceding CDs and digital downloads, record releases were on vinyl (12” polyvinyl disc).

Radio DJs used LP and EP (extended play) to differentiate between a full-length release (2 ‘sides’ of continuous content-usually between 8–12 songs) and Extended Play single releases (one song on a side of a record, usually A side containing the original version of the song that would eventually appear on the full-length LP when the album was completed), while the “B Side” or reverse side contained one or more alternate versions-usually a shortened edit for radio airplay or extended remixes, or vocal only (“A Capellas”).

EPs for artists these days are pre-album LP) releases which are usually devised as marketing tools to anticipate a full release. In the case of newly signed artists record labels release singles or EPs to test market the artist to find out where the target audiences and regions are demographically speaking, so that the full length LP/album release is carried out more efficiently/cost effectively. Another benefit for labels is that there is less production cost associated with a limited EP release and sometimes artists are signed to mini contracts on the contingency that if the initial EP does well the label will fund the remaining album budget and carry the full-length LP release to market.

terrific link at bottom…

1940s
Vinyl record formats: Columbia Records’ 12-inch (30 cm) Long Play (LP) 33⅓ rpm microgroove record versus RCA Victor’s 7-inch (17.5 cm) / 45 rpm Extended Play (EP) during the years 1948–1950. Ended in a compromise because each format found a separate marketing niche, and record players were redesigned to use either type. Both formats nearly disappeared with the rise of the Compact Disc, though vinyl records are still used by niche audiences such as disk jockeys and audiophiles. Also, many newer albums have seen special limited edition releases on LP format.

http://en.wikipedia.org/wiki/Format_war