Shorthand: Furball arena with limited plane set.
Slightly more detailed: Plane sets are (oddly enough) set to Axis planes vs Allied planes, with the rare exception being the Finland setups (since Finns used loaned Brit/US aircraft to battle the Russians as well as German). Generally what you get are balanced aircraft match-ups.
Before the MA split, the AvA was the haven for those that liked to fly early war aircraft without having to worry about being jumped by Spit XVIs, LA-7s and N1Ks all day. The inclusion now of EW and MW arenas have cut down on the usage in the AvA, but there is still something appealing to many about not having constant Spitfire vs Spitfire dogfights.
Killshooter is generally off (so watch your fire as you hurt friendlies), vulching is frowned upon, base capture is useless, and most (can't say all) try to keep it a more gentlemanly atmosphere than you will find in the MA. Oh-and there is no such thing as a "perked ride" in the AvA, so don't be concerned with perks or score. Most folks are there for a good fight, nothing more.