Original Video Animation, it's basiclly a miniseries, higher quality animation because they dont have stretch their budget for a whole series, not as high quality animation as a movie, i know they tend to be more graphic, i dont know if they even air on japanese tv or if they are straight to DVD, might be late night tv in japan
Actully The term OVA came out way before DVD hence the name in its Title orginal VIDEO Animation..
Case in point Mobile Suit Gundam 0083 The Video Stores would get like a New Episode released on VHS every other month for like a Year untill the series was complete..
I tend to think of OVA as Direct to Video Home Movies not a TV Series not a Theaterical release but something in between