cement v. concrete

Concrete is a building material containing an aggregate of sand, gravel, and crushed stone bound together by cement, which accounts for about 10% of concrete’s mass.

Cement is a powder made by heating a mixture of limestone, calcium, silicon, iron, aluminum, and other ingredients in a kiln, then grinding the resulting substance, clinker, with gypsum. When water is added to cement a chemical reaction known as hydration occurs, making the cement harden.

wiseGEEK on cement v. concrete

Wikipedia on concrete