XMPP, formerly known as Jabber, has been around since the late 90's. It is a messaging protocol that has been used in many different messaging apps, such as WhatsApp, Kik Messenger and Zoom to name a few. It is an extremely powerful and robust standard. The XMPP network is decentralized, which means that, unlike WhatsApp, anyone can run an XMPP server, and allow communication to other servers. It is censorship resistant as well. XMPP address are in similar form to e-mail addresses. ie: user@servername.com. Below, I will focus on one particular project, that provides both a server and a client, known as Snikket.
Snikket - a selfhosted XMPP server you can set up with your friends and family.
Snikket is both an XMPP server and a client. The project aims to make it easy for anyone with some basic linux server knowledge to host their own XMPP instance. Even if you don't have the ability to host it yourself, if someone you know does, they can invite you with a simple link, and you can get started chatting on XMPP fairly quick.