Jita Planet 4, Moon 4, Caldari Navy Assembly Plant to be precise. Jita has become the most important, populous and active system in the game, and it's not something that the developers created or designed it has emerged as a trade hub all due to players actions.
The Latest Economic report from Dr. Eyjólfur "Eyjo" Guðmundsson confirms that The Forge as a region now handles over 50% of all the trade in New Eden, all the other regions combined is less than in Jita. As a capsuleer in Eve you will most likely find yourself visiting there, or at the very least asking someone to go there for you if Concord or the Caldari State have taken a dislike to you.
Back in the early days of Eve the system of Yulai sat at the centre of the network of the network of 'superhighway' gates that could be used as shortcuts across the universe. As such for a time it was the most convenient place for trade and Yulai was the center of everything, so much that the system got too busy and CCP felt that the load on this one system was too much for their servers to handle. And so in the Cold War release the superhighway gates were removed and the regional trade hubs of Jita, Amarr, Dodixie and Rens began to take over.
Of course, the developers didn't solve the server load problem, they merely relocated it to a new hub, further developer work on the server load problem would concentrate on reducing the load and coping with it better rather than trying to break up these self organizing hubs. Gates have been added around Jita and the route plotting options gained the ability to avoid certain system, with Jita as a default. All the asteroid belts were removed, all the agents travelled to new homes and no agents will ever send you into Jita on some mission. The hardware people at CCP bought the biggest baddest server they could find, shoehorned as much RAM as they could into its case and then forced it to work 23/7 supporting the most vital system in New Eden. And on top of this there's been continual incremental improvements to make the game code more efficient.
So today, I hear that there's a hard limit of 1400 pilots allowed in Jita, after that people start having to wait at the gates for other people to leave. However we know from experience that it is possible to stuff more pilots into systems without the game crashing, but the results aren't exactly pretty. During the battle for D-GMTI over 1600 pilots were in the system fighting for control of the station, CCP developers were in the system too, observing the back end and no doubt massaging the code to keep things alive.
Jita is important to players as a trade hub, but it's also important to developers as it gives them a place to really examine how the system behaves under stress, even if you never go to Jita you're seeing the effects all through the Eve universe.