The propulsion mechanism for the spacecraft is photons striking the structure of the ship, which is a very large aluminized Mylar surface area made up of four triangles forming a giant kite-like body. When photons strike the sail-like surfaces they will transmit momentum to the ship, and propel it. Because the spacecraft is powered by sunlight, it will require no onboard fuel.
This was designed by the Planetary Society. Japan plans a launch of a similar spacecraft in 2010.