A bipropellant is a combination of two chemicals that are kept in separate tanks and burn in a rocket engine to give thrust when combined.

Fuels that self-ignite when mixed, they are termed hypergolic, most fuels require an ignitions source however, and most will only burn in narrow mixture ratios.

Other propellent schemes such as Monopropellents, and rarely, tripropellents are used.

Examples of good biprops include LOX and Kerosene, LOX and Liquid Hydrogen, LOX and just about anything really ;-)