Is this just an expansion to City of Heroes?
City of Villains is a stand-alone title, not an expansion. It may be played without City of Heroes, but there are clear advantages to owning and playing both. If a player only has City of Villains, he/she may only play a villain, not a hero. Also, if players want to build a base for their Hero characters and take part in PvP adventures (outside of the Arenas) they’ll need to get City of Villains.

So you’re saying that there will be lairs and bases?
Yes, in City of Villains all villain characters will be able to create custom designed lairs and bases with an extremely high amount of detail, rivaling that of the character creation for which City of Heroes is well known. The base-building feature from City of Villains will also carry over to City of Heroes members who have active COH subscriptions, enabling them to also construct hero bases.

How will City of Heroes and City of Villains interface with one another?
City of Heroes and City of Villains will share the same universe. A majority of the new features and zones introduced in City of Villains will be available to City of Villains customers only. Also, many of the existing (and yet-to-be-released) features and zones within City of Heroes will remain exclusive to City of Heroes customers. There will also be “overlapping” zones in which heroes and villains will interact with each other.