'ICQ' was the first 'Instant Messenger' - its similar to MSN Instant Messenger, AOL Instant Messenger and Yahoo Messenger.
'ICQ' is short for 'I Seek You'.
Its main advantage over other instant messengers is the search facility - you can find like minded individuals all over the globe - and people can find you to, if you want them to - just fill in all the forms with your hobbies and interests.
A new feature is the free SMS service - you can sent txt messages to most mobile phones using the software.
Its a good way to keep in touch with friends - like email you can send them a message when they arent online, but unlike email you can have a conversation in real time - and even invite other friends into the chat like a conference call. It'll even tell you, in real time, which of your friends are online.
Once you've installed the software, follow the instructions to register - after registering you will get a 'UIN' - unique identification number. Write this number down - and you can enter it in the 'ICQ' box in this forum's User Options. Next to your name on posts you will have the ICQ icon (a flower) - which will be green if you are online at that time, or red otherwise.