I occasionally read The Triangle Grammar Guide (because I’m just that weird); today I found this article and found out that I’ve been wrong about this for a long time:

Concrete thinking

One of our Home & Garden columnists reminded me recently of the difference between concrete and cement. Cement is the dry powder that is mixed with other things (water, sand, gravel) to make concrete, which is the hardened stuff used for sidewalks and some roads. Cement is only cement as long as it’s dry. Another way to think of it is that cement is to concrete as flour is to bread.

If you are interested [in] a more technical (and rather interesting) explanation, here is one from the WiseGeek.

I always thought that concrete was the generic term and that cement was a brand name, like Kleenex(tm) and Q-tip(tm) .

You learn something new every day.