THERE ARE TWO BTS LINES:
SILOM LINE runs west to south, between the National Stadium in the Siam shopping area and Bang Wa in Thonburi (across the Chao Phraya River).
SUKHUMVIT LINE runs north to east from Mo Chit to Bearing. The two lines meet at Siam Station, and also connect at two points with the underground (MRT) – at Sala Daeng Station (Silom Line) and Asok Station (Sukhumvit Line). A new train arrives every 3 - 6 minutes or so between 06:30 and midnight. The last train leaves between 23:30 and 23:50. Fares start at 15 baht for one stop (more info about the BTS Passes).
Note that trains can get pretty full during peak hours (07:00 - 09:00 and 16:00 - 19:00), as the BTS has also become the choice mode of transport for people living and working in Bangkok.