Raku is an ancient Japanese firing technique, centuries old. It was used as part of the traditional tea ceremony - if a bowl survived the harsh treatment of being pulled red hot from the fiery kiln and doused into water, it was worthy of being used for tea.
I don't do that. I pull my work from the kiln at about 1800 degrees Fahrenheit, then put it into a metal container (which looks remarkably like a trash can) partially filled with combustible stuff, like shredded paper, sawdust, leaves and such. When hot pot meets dry flammables, flames appear.
The main ingredients of fire are heat, fuel and oxygen. When the metal container is closed, the fire is starved of oxygen. The fuel is still there, the heat is still there, so oxygen is chemically pulled from the glazes to keep that combustion going. This is a really simplified explanation of a reduction atmosphere, and is the highly scientific reason you can get a myriad of colors from a single glaze.
Now lots of people say that's not Raku - and they're right. Strictly speaking, it's not. But it is a modification of that process, it does have similarities, and where I come from, It's being taught to kids as Raku. So if you disagree or think I'm being overly simplistic, that's what email is for.
I also do some pit firing. Just as Raku has a rich Asian history, pit firing has its roots in the early native American potters. I fire at the beach, digging out a pit in the sand and lining it with sawdust and manure. Then the work is stacked in the pit, together with wood, charcoal, copper and salt. The fire burns for about 4 hours, and the work is removed once it cools. 
You can find out more about these processes by going to my resources page.