An antimatter rocket is a proposed type of rocket that uses antimatter as its power source. There are a number of possible designs. In one, antimatter would be injected into a target of normal matter and the resulting reaction products (mostly charged particles) would be vented through a magnetic nozzle, providing thrust. A more conventional approach would be to inject the antimatter into a tungsten block, producing heat, which could then be used to either directly heat a propellant (similar to how a nuclear thermal rocket operates), or generate electricity to run some other propulsion mechanism. Finally, a nuclear photonic rocket involves heating up such a tungsten block or other material so it emits light - a curved mirror would then reflect the photons backwards and thus accelerate the spacecraft forwards.
The advantage of this type of rocket is that antimatter, when it annhilates with matter, converts its mass to energy completely. This reaction has perfect efficiency, and so produces far more energy for fuel consumed than fission or fusion. For example, an antimatter rocket could take one ton of cargo to the interplanetary space using about 400 micrograms of antimatter, or to the low earth orbit using half of that amount. For comparison, 95-98% of launch weight of modern chemical rockets such as Zenit or Saturn V is taken by fuel and fuel tanks.
The chief difficulty is obtaining the antimatter, and to a lesser extent storing it. A few atoms of antihydrogen have been produced using particle accelerators, but this method is too expensive to be practical at this time. An additional difficulty is that most of the energy of matter-antimatter reactions is released in the X-ray and gamma ray spectrum, which require exotic mirrors similar to those used in nuclear weapons and x-ray telescopes. Another possible solution to this challenge is the redshift rocket.