Icecap Chat Client Protocol
Shortly put, Icecap is to instant messaging what IMAP is to mail. You can safely exit your Icecap chat client, and Icecap server just keeps on going. When you connect back with your client, you can see all the discussion that happened while you were away (and even older discussion, too, if you want).
You can also use multiple clients even at the same time, so you could for example keep one client always open at home and still be able to check what's going on with your mobile phone.
Icecap clients are encouraged to keep their configuration in server side, so nothing should prevent you from temporarily using another Icecap client from your friend's home with no other configuration needed than Icecap server's address, your username and password.
Icecap is primarily designed as a protocol for implementing a fully featured client-server based IRC client, but it's also designed with other protocols in mind, such as SILC, MSN and ICQ. Although Icecap has some similarities with protocols such as Jabber and Telepathy, it has one major difference. Icecap is a protocol where a thin client talks to a very featureful server, while Jabber and Telepathy are primarily about giving access to different instant messaging networks under a single protocol. They also dumb down the protocols enough that many protocol-specific features get lost (IRC especially).
Note that Icecap isn't trying to replace IRC servers. Icecap server simply connects to normal IRC servers just as a regular IRC client. Icecap protocol isn't either meant to be used as a server-to-server protocol for transferring messages.