When you download and install the client software from Mirabilis, LTD, you answer a few, simple questions about your setup and how you connect to the Internet and a little about who you are. After that, you are given a Universal Internet Number (UIN) that uniquely identifies you on the ICQ network. Other ICQ members can then add you to their contact list via this number, your nickname, or other information you may have given ICQ when you register. I can send ICQ-enabled friends brief messages and/or web site addresses (which can automatically get added to your bookmarks or be loaded into your web browser with the touch of a button). I can also jump to their homepage or directly send them email from ICQ. If the user happens to be online at the time (which ICQ will notify me of, if I so choose), I can also send them files and chat interactively with them in either a split-screen typing session or in an IRC-type screen. You can have also have multiple people talking at the same time (we had five at once one time).
The purpose of ICQ is to enable quicker and easier access to those you want to talk to online, and it does a very good job of that, once you get the client up and running with your own UIN. Installing the client was pretty straightforward, but it took a few tries to successfully register with the ICQ server at Mirabilis the first time -- but I think most of that had to do with the bugginess of a previous version of the client. Once I was able to get a UIN, the Contact List Wizard walked me thru adding my friends to my contact list so that I could communicate with them via ICQ and even gave me the option to email others that might want to get a copy of ICQ to communicate with me.
Design-wise, ICQ makes better use of network resources. One of the problems with IRC and most other "chat-like" services is the heavy dependence on servers. Each user connects and communicates thru a server. If a server in the chat network goes down and your friends are connected to it, you're "split" from them and can't communicate with them anymore. On the other hand, once a user "registers" with the ICQ network, most communication between users will happen directly. The server is only used in "offline" situations or to notify when a specific user has come online. This "lag" problem is, therefore, virtually eliminated. However,
The privacy features of ICQ really set this product apart from any other "chat" system. Aside from the usual "ignore list" you get on IRC, I can require users to be authorized before I can be added to their notify list. The notify list allows you to keep track of certain users and be "notified" when they come online. By forcing people to get authorization before I am added to a notify list, I can be sure that only the people I want to talk to will know that I'm online and everyone else will have to guess the old-fashioned way. I can also restrict it so that I can only receive messages from people on my notify list and block any attempts at wwpages (which allows non-ICQ enabled people to send messages to people with ICQ).
ICQ won't replace your other communication means like IRC or email, but it certainly enhances it. ICQ is currently released for Windows 95, NT (3.51 and 4.0), and Windows 3.1. Macintosh and Java versions are now available as well, but they aren't as full-featured as the other versions (though I have to say the Java version is coming along nicely). The client is considered "beta" (Mac and Java are "Preview") and is freely downloadable and usable, at least for now. Mirabilis hasn't set a price for the "released" version of their software, whenever it comes around. Hopefully it will be reasonably priced, because this one's a keeper.
The protocol ICQ uses has been reverse-engineered and there are now
a wide variety of alternative clients, most of which are for Linux. See
this page for more details.