Literally, "to comprise" is "to include" or "contain." The earth comprises rocks (it includes them), but rocks do not comprise the earth (they do not include it). Therefore:
The Union comprises 50 states.
The whole comprises the parts, but not vice versa.
Strict writers say that using "comprise" in the passive ("One foot is comprised of 12 inches") is unacceptable; instead, use "is composed of" or "is made up of."