XMPP (formerly known as Jabber) is an open and extensible communications standard (IETF - RFC 6120 to 6122). As the name suggests it can be extended by new protocols for new features. In its core its federated and decentralized (just like E-Mail) and in theory any client supporting XMPP can be used (Tho some are outdated for todays features). Some of these are audio / video calls, Multi-User-Groups (MUC, aka group chats), file uploads, vCards, etc. You can check our compliance with conversations.im implemented XEPs here.
There is a variety of clients, the most recommended ones are usually Dino, Gajim for Desktop clients. For a full list see here.
Get yourself an account here, by sending a mail to someone with the handle/nickname you want, to make sure its not being automatically recreated by bots. Your adress will be formatted like this then ‘[nickname@xenofox.nexus]’.
Once you have your account, you can connect with your client to the server to do that, enter your user’s XMPP-Address and password (which has been seperately sent to you). Now you can add other people to your contacts by using their address (which may also be on a different server) or join any MUC. Note: MUCs are usually on a different subdomain as the actual XMPP server, for us it is conference.xenofox.nexus.