The “new” Mo Chit bus terminal was built in the late 1990s to replace the old terminal, which was torn down to make way for the main Skytrain yard. Although technically the Northern Bus Terminal, for all routes north and northeast of Bangkok, you can in fact get a bus from here to just about anywhere in Thailand, north, south, east or west, although service to the others areas is less frequent from Mo Chit than other terminals, so you're recommended to use them if possible.
A typical state-run bus at Mo Chit terminal
The main terminal building is quite large, covering two floors, most of which are filled with ticket booths for the various destinations. The destinations are all displayed on the booths, but there's not necessarily any order to them, so if you're having trouble finding the booth for where you want to go, there is usually someone near the main entrance doors who can direct you to the right counter. They're all numbered, so once you have the right number you should be able to find the right booth.
Buses are run by a number of different companies. The largest is the state-owned Transport Company, which operates buses to just about every destination in Thailand. They're usually the cheapest option, but the buses may not be the nicest ones. They can be fine for a short trip of one or two hours, but for longer trips you're better off with some of the private companies which offer a higher level of service. The buses will be nicer, food may be served on board and some of them even have wi-fi.
Inter-city vans for service to nearby cities at Mo Chit terminal
There are also van services operating out of Mo Chit. These use small passenger vans rather than buses and mainly provide service to the outer suburbs of Bangkok