http://cg.tutsplus.com/tutorials/blender/introduction-to-smoke-simulation-in-blender-2-5-day-1/

In this invaluable, in-depth series we’ll be taking a look at smoke and fire simulation techniques within Blender 2.5, and who better to take us through this process than the man behind the official documents himself, Gottfried Hofmann. Prepare to be amazed at just what Blender can do for free.