The who what when where why and how of fire.

Ok, from my limited understanding, I gather that fire, is the release of energy, but, what causes this release, how does the increase in temperature (and therefore decrease in density?) cause a material to combust? What exactly is fire, how does a spark cause something to combust vs the increase in temperature? any information on this subject would be greatly appreciated. Archived source: https://web.archive.org/web/20091016030030/http://www.spacetimeandtheuniverse.com:80/general-physics/3050-who-what-when-where-why-how-fire.html