Okay, Chico, I need to point out a minor error in what firemanjim said (above)...just in case.
Running nitrous will put you in "fuel" class, yeah, but adding a turbo or supercharger won't necessarily do that -- but either WOULD put you into "blown" class.
Gas (class): You run event gas, which you buy at the track from the vendor, arriving at the fuel truck with an empty tank which the vendor fills and seals. If you get into impound the inspectors will check the seal and sample your fuel to make sure you didn't subvert the rules and add something like Gatorade or hydrazine or whatever.
Fuel: You run anything you choose -- nitrous, nitromethane, bug juice, window washer fluid, event gas, gas from your weedwhacker. No sampling, no sealed tank. Note that you can run event gas in fuel class -- if you see a soft record in fuel class and your motor is happy with what you've got in the tank, switch classes and go make a run.
Blown: Forced induction -- turbo, supercharger, or vacuum cleaner fan. A ram air funnel with internal vortex generators does not put you into blown class.
Unblown: No forced induction. There's been lots of discussion on the fact that some folks refer to nitrous as "blower in a bottle", but the rule stands -- the addition of nitrous oxide induction does not put you in blown class.
Event gas at Bonneville is supplied by ERC and is available in five or six "flavors" - some leaded (110 octane up to at least 117), some unleaded (octanes in the 102-104 range, I think). Rick Gold (the ERC guy) also dispenses alcohol and nitromethane (but DOES NOT HAVE nitrous) and will give advice on fuel chocie if you're not positive what you need. For the last year or so a vendor has been showing up with nitrous for sale, and at least a couple of racers bring "mother" bottles and will fill your bottles for you if you ask real nice.
Just a technical point there, Jim.