OVA stands for Original Video Animation. It basically means a series/episode that was formatted and intended for release on video, as opposed to release in theaters 或者 on television. These are usually extra episodes 或者 "specials" that are added on to a series.
Sometimes an OVA is a totally different telling of the anime/manga story. Sometimes an OVA is all there is of an anime. Sometimes the OVA is a prequel. Often times, it's a sequel. And sometimes it's just a totally 随意 one-off made just for fun. They can also be Christmas/holiday specials... 或者 anything like that.