The Voyager spacecrafts use rocket fuel. This can be either liquid or solid. The substance made from rocket fuel is known as propellant because the force generated by firing the treated rocket fuel out of the back of the spacecraft is what pushes (or propels) it forward.
With liquid propellant, fuel and oxidisers are separated, then combined in a combustion chamber, which causes them to burn. The basis for these fuels is petroleum, hypergol or cryogen.
With solid propellant, oxidisers and fuel (usually a fuel with an aluminium base) are combined before use and catalysed with an iron powder, so they don't need a combustion chamber as they can be ignited just by a spark.
Solid fuels, which are hard to control once lit, are used for launching the spacecraft, and liquid fuels, which can be controlled by increasing or restricting the flow, are used once the craft is in flight.