ICQ (unless there is another "ICQ" that i've never heard of, which case i'm off in left field) is an instant messenging system similar to AOL instant messenger, Yahoo messenger, and the like.
i'm not sure but i think it's actually one of the most long lived of the systems (ie came on the internet scene early on).
beyound that i can't remember any details. having myself only dabbled with ICQ for a short time when a (now) former girlfriend used it. i do remember that it was much more cumbersome than the other IM systems requiring (at that time) a sort of "account number" of the person(s) that you wished to send messages to (it saves them of course). the trade off for the clusy interface (for adding new 'friends") i remember was that it made ICQ just a slight bit more "exclusive" in the area of who could send you a message "out of the blue". in my case as soon as the GF in question was out of my personal picture the difficulty of use caused me to dump ICQ along with the girlfriend.
due to the relative clumsiness of use, i'm not sure why the system is still around. except for a very few people scattered about on the various webForums i freaquent, i no longer even see much reference to ICQ, Yahoo, MSN and AOL messengers, being MUCH easier to use and having taken the lead in the IM realm.
but maybe it's a case of whatever you start out with seeming the best system, and maybe ICQ users find it easier to use than the others. ~shrug~