Oh well, I'll figure it out soon enough.
Yeah, Microsoft didn't do the buying public any favors with their confusing names.
Generational progression:
XBox
XBox 360 / 360s
XBox One
XBox One X / One S
XBox Series X / Series S
Wiki has a decent features matrix by generation. It's about a quarter of the way down the page.
https://en.wikipedia.org/wiki/Xboxdogmush is right... games optimized for the XBox Series X won't play on previous-gen consoles unless the developer offers a backwards-compatible version, an expensive and time-consuming proposition. It is, in a very real sense, developing two different versions of the game to accommodate the huge differences in hardware architecture and raw processing power.
Fortunately, the XsX will play most older games, some of which are available in reworked versions with fresh skins, new overlays, and improved physics that weren't possible under older consoles' processing limits.
As a die-hard A/V enthusiast and occasional gamer who made the jump from XBox One to XBox Series X, I highly recommend the XsX. Yeah, it's spendy, but worth it for the versatility and performance. Plus, it capitalizes on a growing market of products rather than a shrinking one, will be functionally relevant for many years to come, and has you ready for that new 4K TV you've mentioned a couple times.
Brad