What is the best way to draw explosions, clouds and smoke? I know I may use particules and billboards, but I want more details...

The best way to draw something like smoke or clouds (and probably the only logical way) would be to do particles which are polys that always face the camera, and their detail depend on the texture you use. For explosions you could do use 3D models but I have to say that most of even recent games, like Tribes 2, still use a well animated particle. If you remember some of the explosions in Quake 2 were 3D models, but in Quake 3 they went back to particles. They simply look much better if the art work is done properly.

There is good tutorial done on advanced particle systems (for Oxygen3D) which you can find on gamasutra. check it out

Hope that helps