Summary: Something that coulda been really cool & awesome, if not for crappy decisions made by the Dark Lord...er, I mean IBM.

Lot more details: https://en.wikipedia.org/wiki/Token_Ring

My local hospital system (Premier Health) was all-in on IBM and token ring. I remember being chagrined when they built a new hospital in 1998 and wired everything for token ring. The writing was clearly on the wall for token ring by 1998 but their IT department bled IBM blue.

Fast-forward to 2013, when my wife was giving birth to our daughter in one of their hospitals, and I noticed an unused type-1 token ring wall jack in our room right beside some occupied 8P8C (aka RJ-45) jacks.

Token-passing networks were interesting for hard real time system analysis, as they featured deterministic access control instead of collision detection. They made possible extending Rate Monotonic Scheduling theory to the network layer as well. The priority work (priority ceiling protocol, multi level token priority schemes) survive today in token bucket rate limiting.

The "WAS" in that title just made me feel really old. I remember learning about this in college and it being a thing that I would need to work with and understand how it works why it's different than ethernet. And now it's just a big "WAS".

Short version:

Token Ring was a networking alternative to Ethernet.

With Ethernet when a node wants to transmit it just does so, then backs off and retries if there was a collision (someone else trying to transmit at the same time).

Token Ring avoided collisions by passing a "permission to transmit" token from node to node in circular (ring) fashion.