There are three main bus stations in Bangkok with coaches and minivans departing to all areas of the country and beyond.
The Northern Bus Station
The Southern Bus Station
The Eastern Bus Station.
This is Bangkok's biggest bus terminal and if you want to travel by bus from Bangkok to the north, north-east or central areas of Thailand, Mo Chit Bus Station is the place to leave from.
There are also a few departures from here the some eastern cities, as well as some southern cities.
This is also the best place to catch a bus from Bangkok to Cambodia and Laos.
If you want to travel by bus or minivan to the southern cities, beaches and popular islands in the south of the country, Sai Tai Mai bus station has the most departures to places like, Hua Hin, Phuket, Krabi, Trang, Hat Yai and Surat Thani to name just a few destinations.
You will also find departures to the west such as Kanchanaburi.
The smallest of Bangkok's Bus Station, Ekamai Bus Terminal is located in the heart of the city, along Sukhumvit Road and deals mainly with buses and minivans to the eastern coastal towns such as Pattaya, Chonburi, Rayong and Trat, as well as buses to get you to the islands of Koh Samet and Koh Chang just off the eastern coast.
You can also get buses from here to Aranyaprathet / Poipet (on the Thai / Cambodia border).
Not a bus station, but a very popular departure point for buses and minivans to many of the most popular tourist destination is around the Khao San Road area.
If you are staying in this popular backpackers area of Bangkok, catching a bus from here is very convenient.
Popular buses leaving from the Khao San Road are to Chiang Mai, Koh Samui, Koh Phangan, Koh Tao...
Bangkok's main minivan departure point at Victory Monument is no more and now most minivans / minibuses have now been relocated and will depart from the respective bus stations.