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 o on television. These are usually extra episodes o "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 misceláneo one-off made just for fun. They can also be Christmas/holiday specials... o anything like that.