In the hotrod world, what is the diffrence between a fueler and a gasser?

In simple terms, a "gasser" is a hot rod that runs on gasolene (petrol here in the UK) whereas a "fueler" is a hot rod or dragster that runs on nitromethane, which may or may not be mixed with methanol, and the vehicles are usually referred to as "Top Fuel" and are usually "dragsters".

A fueler or fuel dragster runs on nitromethane or mixture of nitromethane and methanol. This fuel is used on extremely high-powered, long-wheelbase machines built specifically for drag racing.

A gasser runs on gasoline (petrol). This drag-racing class was popular in the 1950s through the early '70s but are no longer a major competition class of the National Hot Rod Association. Unlike purpose-built fuelers, gassers are mostly modified production cars, often older coupes and sedans, such as Willys, Tri-Five Chevys, Ford Anglias, etc. They have high front ends and straight axles which are far less safe at high speeds than low-ended fuelers which use nothing like a conventional car's suspension.