A chat net or IM net or instant message network is just a network of terminal devices, each with an IP number usually, that is used by chat code to set up interactive text conversations between users.
Because it also integrates easily with pager and radio broadcast and even voice technology, chat nets will almost certainly be at the core of any healthy signal infrastructure that relies on people moving around.
The ICQ network is almost an ideal chat net since it does not store one's contacts on a central server, and so they remain private only to the clients. This can be accessed by Trillian, Jabber and ICQ's own client. Trillian is the ideal front end under Windows because of its 128-bit encryption over ICQ, and the fact that you can set up an ICQ account in one click. This is enough security for any healthy buying infrastructure.