XD / XDR
XD and XDR are SRAM's freehubs that allow a 10-tooth minimum cog. Instead of splines, the cassette threads onto the body as a single piece. XD is for mountain (SRAM Eagle); XDR is the road/gravel version, identical but 1.85 mm longer. An XD cassette fits an XDR freehub with a 1.85 mm spacer; not the other way round. Neither is compatible with HG or Shimano's Microspline.
If you run SRAM Eagle (MTB) or SRAM AXS (road), your wheel has XD or XDR. The difference between them is 1.85 mm that confuses half the world.
What they are and how they differ
Instead of splines, the XD/XDR cassette threads onto the freehub body, leaving the 10T cog overhanging the end. XD was born for SRAM's 1x (1x11, 1x12 Eagle). XDR took the same idea to road and gravel, lengthening the body 1.85 mm to respect road wheel spacing. Externally they're identical; only the length changes.
Key data
What it fits
SRAM mountain cassettes GX/X01/XX1 (11sp and 12sp Eagle) on XD; SRAM road/gravel AXS cassettes on XDR. An XD cassette fits XDR with a 1.85 mm spacer; an XDR road cassette won't fit a mountain XD (too short). Nothing compatible with Shimano HG or Microspline.
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Frequently asked questions
What's the difference between XD and XDR?
Only the length: XDR is 1.85 mm longer. XD is mountain; XDR is road/gravel. An XD cassette fits XDR with a 1.85 mm spacer.
Do I need a special tool for an XD cassette?
No. It comes off with the same standard Shimano/SRAM cassette tool (Park Tool FR-5 type).
Does an XD cassette fit an HG freehub?
No. Different interfaces (threaded vs splined). XD exists precisely to allow the 10T cog that HG can't take.