whats the difference between OVA and the anime aired?

Well, you probably already know what the definition of an OVA is, but it can be multiple things:
- A side story of an anime
- A prequel or sequel of an anime
- A short story almost completely disconnected from the anime itself
- Fanservice
- An anime series that simply didn't air on television, but stands on its own

If I like an anime, I'll usually check out its OVA if it has one. I will warn you though, if you watch the OVA's for every anime you watch, you will eventually encounter some major wastes of time. It all depends on the show, though.