What is the difference between a normal dump truck and a tandem dump truck?

A tandem dump truck is as normal as any other dump truck. Tandem simply indicates that it has two drive axles (as opposed to a single axle, which only has one). A tandem dump is typically allowed to gross at 54,000 lbs. From there, some dump trucks have additional axles (lift axles) which allow them to haul more weight. They're named in accordance with the number of axles behind the cab (i.e., not including the steer axle). So a tandem dump with a single lift axle would be referred to as a tri-axle, with two lift axles it would be referred to as a quad axle dump, with three lift axles it would be referred to as a quint axle dump.... at four lift axles, they're typically referred to as 'centipedes'.