When the Magic Kingdom was being designed in the late 1960s, it was decided to put the parking lot far enough away from the park itself that it couldn't be seen, in order to assist in the "suspension of disbelief." To accomplish this, the Magic Kingdom was built on the north side of the completely man-made Seven Seas Lagoon, and the parking lot was built on the south side, 3/4 of a mile away and completely out of sight from the park.
To get from the lot to the park, one needs to take either a ferryboat directly across the Seven Seas Lagoon, or a monorail around the Lagoon. With these two transportation options, stations or terminals were needed, and so the monorail stations and ferryboat terminals on the south (lot) side of the Lagoon were built together, forming a complex of sorts. Ticket Sales windows were added, as well as a bus depot and several other transportation-related facilities, and the complex was dubbed the Transportation and Ticket Center. In 1982, when Epcot was constructed, another monorail line was added from the TTC to Epcot, with a station at TTC built next to the original two.
TTC was originally envisioned as the main transportation hub for any kind of transport in and around WDW, and still serves as the biggest transportation hub. However, as new parks and resorts were added to WDW over the years, and priorities changed, TTC has not remained the end-all, be-all of transportation that it was in the early days.
At TTC you can still find the following facilities (and I may leave some out):
1) The Magic Kingdom's parking lot
2) WDW Ticket Sales windows and Guest Relations windows
3) Monorail station for the Magic Kingdom Express line (which only stops at MK and TTC)
4) Monorail station for the Magic Kingdom Resorts line (which stops at MK, TTC, and three MK resorts - Polynesian, Grand Floridian, and Contemporary)
5) Ferryboat terminal for the Magic Kingdom ferryboats (General Joe Potter, Admiral Joe Fowler, and Richard F. Irvine)
6) A bus depot serving busses to Disney's Hollywood Studios and Disney's Animal Kingdom parks, and Orlando's Linx bus system (WDW resort buses do not stop at TTC, they go direct to MK)
7) Pet kennels
8) Footpath to the Polynesian resort
9) Taxi drop-off area
I'm sure I'm leaving some stuff out; TTC is still a pretty busy place, despite the sprawl that WDW has undergone since it was initially built.