Difference between Cilantro and Coriander

We had a discussion in our family the other day about two words: CILANTRO and CORIANDER. I assumed they were different herbs altogether, however I'm not sure. When I go to the supermarket, I see leaves labeled cilantro and another kind labeled coriander. Is there a difference?

Cilantro is a form of coriander. It refers to the fresh leaf form of coriander that is used fresh in soups, stews, etc. Coriander seeds are from the same plant but have a very different role in cooking.
In recent years, different coriander varieties have come out suited for either the fresh leaf usage (cilantro) or the spice seed usage. The leaf corianders are slower to go to seed and tend to produce more leaves before finally flowering and setting seeds. Once coriander begins to flower it stops producing leaves which is undesirable if you want fresh leaves only.
There are other corianders. Vietnamese coriander and mexican coriander are two, and there are others. Each offers special features that are not found in the others. For example, vietnamese coriander never flowers and only produces leaves, great for cilantro lovers.