The Metropolitan Rapid Transit (MRT)
The Metropolitan Rapid Transit (MRT) agency runs the Bangkok metro system. It opened in 2004 and currently has 18 stations along 27 kilometers of underground rails. It recently just completed a new line in 2016 and is still expanding its metro lines.
There are two types of tickets that you can purchase on the metro, a single journey token or a card that you can add value to. The price for the single journey token depends on what station you get on and off at. There are different types of cards you can purchase, with discount for students and different age groups. You can also buy a 24 hour pass for 120baht or a three day pass for 230 baht.
Bangkok Mass Transit (BTS)/ SkyTrains
The Bangkok Mass Transit (BTS) is the transit agency that runs the SkyTrains in Bangkok. They are separate from the MRT. BTS opened in 1999 and currently has two lines, the Silom Line which goes west to south and the Sukhumvit Line which goes north to east. In addition it has the airport rail link to the Suvarnabhumi Airport and has a planned line for the Don Mueang Airport.
There are two types of tickets you can purchase for the SkyTrain, a single journey for 15-52 baht depending on zones crossed, and a day pass for 140 baht.
Map of BTS and MRT system