All starships use Neutronium for fuel. Neutronium is a superdense material found at the core of many planets. It is also known as "neutronic fuel". Fuel is millions of times denser than common elements such as lead or gold. The fuel must be stored within a very powerful inertial damping field to prevent damage to the starship's structure during maneuvering or acceleration. The amount of fuel that a starship can carry is limited solely by the strength of that inertial damping field and not the cargo room available on the starship.
For details on fuel consumption, click here.
Starships without fuel cannot attack enemy ships. Enemy starships are not allowed to attack starships that are out of fuel, due to the treaty of Nimbus 7. The treaty states "No starship may attack or destroy a starship that is out of fuel. Such starships are helpless and unable to protect themselves. Destroying helpless starships would be a tragic and senseless loss of life." Fuelless starships can still be attacked by planets using the NUK Friendly Code. As well, the Horwasp Plague makes no distinction between starships with fuel and those without fuel.
Starships without fuel cannot perform most missions, including cloak. More information on this can be found on the Out Of Fuel page.