"OVA" stands for original video animation. (It's sometimes referred to as "OAV," or "original animated video.") Essentially, this is just what we in the United States call "direct-to-video" (or more recently, "direct-to-DVD"). Anime OVAs can take the form of episodes or movies. They can be one-shots or they can be series both short and long. Sometimes they're related to anime TV series or movies, and can come either before, after, or in between any of those formats. And for fans outside of Japan, it may be a bit confusing, because there's a good chance our first exposure to some of these things was on TV. But... the internet should do a good job of informing you of whether something was an OVA or a TV series, originally.