Jita is a Trade Hub system for Caldari space and Jita 4 Moon 4 Caldari Navy Assembly Plant is the station that the majority of the pilots and traders use in the system.
In fact Jita is the largest, busiest trade hub system in all of EVE, with the average pod pilot population being 350 near downtime and around 1000 almost any other time, up to around 1500 at weekend peak times. Unfortunately this results in major lag throughout the system. The reason why Jita became so busy is actually unknown. However, theories suggest it is because Jita 4 Moon 4 Caldari Navy Assembly Plant was the first destination tutorial agents sent Caldari pilots to after they finished the tutorial. It also was a destination on the travel highways before they were eliminated and took over the lead trade hub from the old central highway destination of Yulai. Despite the fact that it is no-where near the center of the cluster, Jita is more or less in the center of the Rens, Oursulaert and Amarr trade hubs in Empire. The fact that it is very close to several Caldari starting spawns also helps its population figures.
Due to its population, Jita has become the most affected system when it comes to CCP's Need For Speed Initiative, with all asteroid belts in the system, and the busiest gate in the game (Jita -> Kisogo) being removed entirely. CCP recently revealed that Jita is one of only 5 systems in the whole of Eve that runs on its own dedicated server hardware and doesn't share with any other system.
The busiest station in the game is recognized player-wide as Moon 4 - Caldari Navy Assembly Plant and has an average of thirty to fifty ships outside of its docking port at a time. Legend has it that this station was the sole reason for CCP including a station and gate proximity limit on Smartbombs, after assorted groups of players using bombageddons down to smartbomb fitted destroyers, frigates or rookie ships cause a mass appearance of CONCORD in front of said station.
In the Chinese spin-off game of EVE Echoes, Jita remains the busiest trade hub. Its extreme popularity is probably due to its fame in EVE Online.