OVA

The name OVA stands for original video animation. One aspect that makes differ greatly from the anime series is that they are release directly to dvd's, and back in the days they were release in VHS. This creates a great difference because the director has much more time to develop and more detail plot; a normal TV episode needs be between 20-24 minutes and a movie is usually not more than 2 hours; however, with OVA a director can take as much time as she or he needs. Also, the pressure from having to expand a short plot inserting fillers that usually have nothing to do with the original plot or to shrink a huge plot into a TV season disappears. Usually OVA are known to be more faithful the original manga because of freedom in time they can be far more detail and contain much more of the background and motivations of the characters; also, since they don't have to fallow the censorship rule in order to be able air the TV series in a reasonable air time, they OVA can be far more explicit, and violent ( not that they necessarily are). OVAs are not as popular here since they are not release on TV series or movies. They started to be produce in the 1970's, and the first official OVA was name Dallos and it was release by Bandai.

-Source:

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