What are OVAs and how are they different from Specials?
Broadly, the two terms could be interchanged, although there are a few points of (almost) difference:


OVAs are generally released for home video (be that VHS, DVD, or whatever), whereas it's possible for a Special to be released as a television broadcast that is just separate from the rest of the show.
OVAs are often shorter than a normal episode of the show, while Specials are often as long or longer, although this definitely doesn't always hold.
Some OVAs are not associated with a show, so they aren't a "special episode" of anything. For example, the series Video Girl Ai was only ever released as a 6 episode straight-to-video OVA.