CoV is the standalone villains expansion to "City of Heroes". The biggest feature of this game is the vast differences you can create in your character, you create a hero or villain from scratch customising every aspect of him/her with an enormous selection of parts like lizard tails and robot legs. The main criticisms of major mmos is that people always look the same later on in the game because of identical gear but there is no such thing as 'gear' in CoV, you completely choose your appearance and the only person to change that is you.
The basic game doesn't vary much fom CoV - instead of defending a bank against a bank robbery you will be the person robbing it. If you own CoH then you will enjoy the new selection of evil appearances to choose from.
The biggest improvement from this expansion is the inclusion of player vs player combat where good and evil can fight in arenas.
Ofcourse, CoV can be purchased on its own and is a great standalone game. The format is a very simplified online roleplaying game aimed at the casual audience that only has 45 minutes online at once. You enter 15 minute quests with up to 5 friends or strangers and battle against the forces of google/evil. You acquire upgrades to your skills and can trade them with other players in case you loot something thats useless to you.
The game has a great LFG (Looking for group) system that helps you very swiftly find people your level looking to quest together and the community is generally mature and helpful.
A sister game to City of Heroes, CoV is a stand alone game as well as an expansion pack that lets online players take on the role of the world's most evil super-criminals. Choose your powers and try not to use the "dance" emote too much as you spread evil across the digital landscape.