XMPP is a protocol for decentralised chats, allowing people registered on different servers to chat with people on other servers, kind of similar to how email works (and Lemmy of course).

Google Talk was a service by Google which started with XMPP support, letting users from other XMPP servers chat with Google Talk users. Google Talk was always slightly different from the XMPP standard, due to having proprietary code in its backend, leading to chats between Talk users working flawlessly but not between XMPP and Talk users. Slowly, Google Talk became more popular than the other servers.

Eventually, XMPP server-to-server support was removed as part of their transition to Hangouts, meaning once Talk users switched to it, XMPP users would no longer be able to chat with them and would have to switch to Hangouts. While XMPP still exists today, it’s definitely a niche nowadays, and this is part of the reason.

Edit: proper paragraphs

That’s how people thought it would have gone with XMPP and Google Talk, but that’s not how it went at all