Gnu is a public jabber server. Jabber (or xmpp) is an open protocol for realtime communications. In contrast to most similar protocols, it is decentralized. That means that there is no central server, but a network of many equal servers. To some extend, it is similar to how email works.
Every user can freely register on any server they like and trust, in order to chat with users across the whole network.
The above applications communicate with gnu in an encrypted way, same applies to communication with other servers.
For end to end encryption we recommend the use of OTR (Off-the-Record) protocol. All of the above suggested applications support this, either by default or through a plugin.
One of the motives for creating gnu was to build a jabber server that will keep a minimum amount of (meta)data and only the bare minimum that is required for its core functionality, in respect to users privacy.
Data that we do not keep on the server
In order for this service to work, it is necessary to keep these data.