City of Heroes has no open PvP as such. It all takes place either in the Arenas, through the Arena kiosks in Pocket D, via friendly 'instant' base raids, or (most commonly) in one of the four designated PvP zones. Each one of these has a different theme, and features a minigame whose rewards have PvE benefits; often, they are more use for PvE than PvP.
For the benefit of new players, and those who've not explored the PvP zones of CoX to their fullest, we thought we'd take a stealthy trip around all four of them and walk you through how the minigames work. Even if you're not much of a PvPer, it's well worth knowing how to get a Shivan pet, or a one-shot nuclear missile blast, if only because it looks so freaking cool to call down an atomic strike in the middle of a mission.