Used in forum roleplays, to mean a reply. It is usually used as a verb, but can also be a noun. When people just say "tag", it means that they have replied and are waiting for you to tag them again.
v. "Tagging" is the act of delivering a quick, sharp, blow to the scrotum. A well placed tag will leave the victim rolling on the ground, clenching his testicles in agony, possobly even vomiting.
Beleive it or not, some people find the pastime of tagging entertaining.