http://en.wikipedia.org/wiki/OVAThat should probably explain it better than I can. OVAs tend to have better animation due to higher budgets than normal TV shows.
Good. I can't imagine Roberta firing a Barret .50 cal sniper rifle with grenade launcher attachment while in midair in low quality animation.
Also, consider reading the manga.
It's like the difference between the Hellsing anime, and the Hellsing OVA.
Or the GaoGaiGar anime, and the GGG FINAL OVA.
Or Rebuild of Evangelion.
Or the new TTGL movies.