A solar sail traps photons streaming from the sun similar to the way a sailboat uses wind blowing around it. The sail is reflective, very light weight (mass), and extremely large. The solar wind blowing from the sun (sol) impacts the solar sail and provides the force to move the mass of the sail and the boat or instrument package being dragged along for the ride. The friendly folks at the Planetary Society are currently working on launching one from earth orbit. See below.
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A huge sheet of Mylar with a reflective surface. It is like a giant parachute with a spacecraft attached. The idea is that solar pressure will keep the sail inflated and push the spaceship. See, "Sunjammer", by Arthur C. Clark, a great sic-fi book ad out that.