ICQ is an instant messaging service which is owned by AOL. It was originally owned by Mirabilis, an Israeli company.
It was launched November of 1996, and on June 8, 1998 it was sold to AOL for $407,000,000.
Spoken aloud, the term sounds vaguely like "I seek you".
The main difference that users notice between AIM and ICQ is that ICQ issues a Universal Internet Number, or UIN, whereas AIM lets users choose their own screen name. Both AIM and ICQ are owned by the same corporation, Time Warner, so AIM users and ICQ users can freely message each other without the need of having to download any other software.