City of Heroes is an MMORPG in which players create heroes to combat the villains attacking Paragon City. In doing so, they collect experience and become more powerful, to the point where they become involved in more important efforts to save the city.
From the official web page: Super Powered Action and Adventure
Create Your Own Hero: Choose from hundreds of different powers and design your own unique costume.
Fight Evil! Confront super villains, aliens, madmen, criminals, and other fearsome foes. Take on personalized missions and rid the city of several different evil organizations and hundreds of individual enemies.
Form Your Own Super-group of Heroes: Band together with other players to fight evil and become the premiere hero group in the city!
Explore the City: Explore the skyscrapers, slums, sewers and streets of Paragon City, a sprawling online metropolis that offers unlimited adventures and countless surprises.
Live the Story: Become an integral part of more than thirty different ongoing story arcs as the villain groups menace Paragon City and react to player victories and defeats.
City of Heroes and City of Villains, which were once separate games but as of July, 2008, are combined into one.