What is the difference between coriander and cilantro?

Coriander and cilantro are the same plant, coriandrum sativum.  Typically, the seeds are called coriander.  The leaves may be called coriander, cilantro, or Chinese parsley.  Both parts are used in cooking.

The coriander seed is small, about 1/8 inch (3 mm) and is light brown or tan in color.  It has a sweet, slightly anise flavor.  It is used in foods from a number of different ethnic origins, showing up in curry blends, pickling spices and corned beef recipes to name a few.  As a child you might remember eating those small black jawbreakers with a seed in the center.  The seed was coriander.

Cilantro is used in Mexican and oriental cooking.  It looks somewhat like Italian parsley, although the leaves are a slightly lighter shad of green and not quite as thick or rigid as parsley.  Cilantro has a musty flavor that some people find objectionable.  Personally, I found that I had to taste cilantro three or four times before I became accustomed to the taste and actually started using it in my cooking.  Like parsley and arugula, cilantro is sometimes used as a salad green also.