*yanked over from the manga thread*
Original Animation Video & Original Video Animation (OAV / OVA) are interchangeable terms used in Japan to refer to animation that is released directly to the video market without first going through a theatrical release or television broadcast.
Furthermore, OAVs are supposed to have original scripts, although there are exceptions. They can be based on a Manga or TV series, but the particular episode should be original. For example the Ranma ½ TV series follows Rumiko Takahashi's Manga fairly closely, however the Ranma ½ OAV series contains stories that were not in the original Manga series.
OAVs are generally higher quality than TV series because a larger budget is allocated to the production and more time is allowed for the work.
With very few exceptions, the vast majority of all pornographic Anime is released as OAVs since the material is not suitable for theatrical or television presentations.
There.