OVA and OAV are pretty much the Same they often stands for as OVA - Original Video Animation or OAV - Original Animated Video sometimes used but means the same thing.

In Japan, anime is normally released in one of three different ways.

TV
OVA
Film or Movie

When a new anime is created, it is normally aimed at one of these three marketplaces.

TV
TV series tend to be either 13 or 26 episodes long (since a TV 'season' is a quarter of a year long, 13 weeks). Sometimes the exact number of eps vary, so you may see 12 or 25 episodes, or the series is popular so it keeps on running as long as there is interest (and money!).

DVD releases of TV series occur after the series has started, depending when the Marketeers think they can gain the greatest financial reward. Very difficult to say when a TV series would be released in DVD form.

Since most TV series are shown on a weekly schedule, the animation can suffer from being rushed and you also see the occasional 'recap' episodes (which uses clips from previous shows).

A typical TV episode normally lasts for around 23-25 minutes (to allow for adverts).

OVA
An OVA release means that the series hasn't been shown on TV before but is released straight into the 'video marketplace'. That's where the name comes from, it's an original video release (and the term was used many years ago before these new fangled DVDs were popular).

The OVA release schedule can be several weeks or months between one episode and the next (and please note, it is common for just one 30 minute episode to be released on a DVD in Japan). Due to this, the animation for an OVA can be very high quality. OVAs are funded before the project is started so they can put time and money into the story and artwork.

The length of an OVA can vary a lot but 4 or 6 episodes is common (or used to be). I think the general consensus is that the OVA marketplace is declining and so not as many are being made now as 5-10 years ago.

A typical OVA episode lasts for 30 minutes (since they don't have to allow for any advert breaks).

Film or Movie
Lastly we have the 'Theatrical Releases' or Films. These are shown directly into the cinemas first and then have a later DVD release (6 to 12 months maybe). Like OVAs, the animation is of higher quality to TV series, for much the same reasons.

Films or Movie vary in length, they might be as short as 30 minutes or run for a couple of hours.

So in general the type of release, TV, OVA, Film or Movie, indicates how it's first released in Japan and an indication as to the length (number of episodes) you might expect. There are exceptions to the above but in general they seem to follow the same pattern.