XMPP is an IM/group chat protocol that supports end to end encryption, contact lists, etc. Think of MSN Messenger, AIM, and other IM programs that have come and gone over the years. XMPP started in the 1990s and has been developed and improved since then. NHTB currently hosts our own XMPP server that is open to our guys for their communication needs.
You’ll need a client to get started. We recommend Siskin IM for iOS (there is a minor security issue with this client; change the name of your iPhone to not include your name since Siskin uses the iPhone’s name as its resource ID), Conversations for Android (free on F-Droid), Gajim for Windows and Linux, or Psi+ for Windows/Linux. Mac users will have the best luck with Monal IM at this point. If you prefer a web client, NHTB hosts Converse.js and Xabber. Each client’s registration screen will be slightly different, but the following list should serve as a good basic guide.