Lots of MMORPGs have guilds - a way for players to coordinate, gather loot, set up bases and work their way into the context of the game world. Some dedicated World of Warcraft players even do up business cards with their characters names on them, what server they play on, what side of the conflict, and what their guild is.
In EVE: The Second Genesis, CCP's massively multiplayer persistent world online game, the concept of the guild gets a facelift, and is called the corporation. Characters form corporations to lay claim to specific parts of the game universe. They control access, can charge tariffs, and otherwise act with nearly the full range of actions and activities that a real world corporation does. EVE Online corporations are far more vibrant than the usual run of “raid, counter raid, camp on the spawn points".
One of the ways that EVE Online corporations significantly differ from a guild is that they can issue contracts, and interact in the market place as entities on their own. Some MMORPGs allow this to a limited extent, but nothing to the scope that these corporations do. Because the entire universe in EVE is persistent and dynamic, changes that corporations make to the basic demand curve for commodities matter. EVE Online Corporations can, in the finest black-hat corporate style, create a spike in demand for a product, and hire out for contractors to get the product - while also paying for pirates to raid the shipments.
EVE Online corporations have a span and a scope that's unique in MMOGs, and can rise, fall, and rise again from the ashes in ways that become part of the ongoing background to the EVE Online game.
What makes this possible is that EVE doesn't run on multiple servers; it runs on one supercluster computer for each instance of the game (Tranquility, for English speakers, Serenity for Chinese, and Singularity for testing new code roll outs). Because of this persistent universe, all 220,000 of the players are interacting with each other; because of the ratio of “player versus player" enforcement inherent in the universe (with 5,000 star systems, to boot), EVE corporations fill a wide range of services and niches beyond just manufacturing. Corporations make loans to one another, provide escorts and anti-piracy groups (and even fund pirates to operate on one another by proxy war).
The downside of EVE Online corporations is the learning curve needed to make one viable. Fortunately, while they add a lot to the game for the hard core audience, these complex corporations can be mostly ignored by players starting out.
Derek Smithson has written articles on EVE Online ships and the EVE Online free trial which is available, as well as a number of EVE Online guides .