Of the city buses in Bangkok you have either the:
(1) BMTA (Bangkok Mass Transit Authority) buses or the (2) Private Buses
This is the BMTS emblem on the side of a BMTS bus.
Bangkok City Buses will include AC or non-AC buses and will come in many colors. The below prices quoted will be for a 'local' run. This price will on average 'doubled' for longe distance runs. I will try to break it down to just a few of the buses. (May 2008 prices)
The 'red' and 'white' buses will be non-AC (local aprox 7 baht.)


The 'blue' buses can be non-AC (local aprox 8 baht) or AC (local aprox 10 baht)


The 'orange' buses are AC (local aprox 12 baht)
There are the short 'green' buses (avoid these at all costs).
There are also the new 'yellow' modern buses being introduced in downtown Bangkok.