A lightsaber resembles a sword, but instead of having a blade made out of metal, lightsabers have a plasma laser beam for the blade, powered by an internal power cell contained within the hilt. This beam is very powerful and can cut through almost anything.
A lightsaber's blade will typically be red, blue, or green. The Sith are known to wield red lightsabers, and the Jedi are known for their signature blue and green lightsabers. In some of the Expanded Universe books, there are lightsabers that appear in a variety of other colors as well.