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Jabber is an open system for Instant Messaging. Jabber uses a protocol called XMPP. It's a distributed system, that means that there isn't one single server or one centralized service where everybody has to have an account. There are multiple servers and providers, just like with email. You simply log on to your provider's server and start chatting with your friends. The servers will take care of exchanging the messages between the providers.
You can connect to servers with a lot of clients, like:
There are lots of servers, like:
Remember that being an open system, you can even setup your own server and become part of the network. For this purpose, you can use some of the open source server implementations:
The communication between clients and servers is protected with TLS. That means that the communication between users and their servers is secret and nobody can read it.
Servers can have additional services rather than just messaging and presence. Most servers have chatrooms (Jabber is not only one-to-one communiation), many also have gateways to open networks (like IRC) and to closed ones (MSN, Yahoo!, AIM).
