From LQWiki

ICQ: an instant messaging program formerly from an Israeli company named "Mirabilis", now owned by AOL. Clients like GAIM and LICQ can use the ICQ protocol.

Because ICQ is owned by AOL ICQ users can send message to AOL users. ICQ includes a comprehensive people search and profile setup that dwarfs that of AIM or the Yahoo IM programs. ICQ is known to be an internationally used IM program.

ICQ is typically used for sending individual messages; if you want to talk with a group of people, you can chose the chat protocol irc.