What is the difference between the BFBC series and BF2?
I only ever played BF2.
BF:BC is basically an console spinoff of the original BF series that started on the PC. The difference with BF:BC and BF2 is mainly updated graphics and a more involved singleplayer experience, but a scaled down multiplayer experience (less players, smaller maps, etc) to accomodate limited console resources. The core gameplay mechanics are largely the same as in BF2 though.
BF:BC2 will follow on from BC, but will also be released on the PC unlike the first.
Lastly, BF3, the proper sequel to BF2 is a separate game that has been confirmed to be in development but will most likely be similiar to BF:BC2 but with bigger player capacity and larger maps.