Cassette
The cassette is the rear sprocket cluster that slides onto the wheel's freehub body and is held by a lockring. It defines two things: the speed count (how many sprockets) and the range (the difference between the smallest and largest cog, e.g. 10-52T). It doesn't thread onto the wheel like the old «freewheel»: it mounts on the freehub, so it must match the freehub type (HG, Microspline, XD or XDR).
It's the part that most changes how you pedal: more range = easier climbing. But range and speeds depend on your wheel's freehub.
What it is and what it defines
A cassette is described by its speeds and its range. An 11-34T (road) has small jumps between cogs; a 10-52T (MTB) covers brutal climbs to flats. More range demands more from the rear derailleur (its capacity). The smallest cog dictates the freehub: a 10-tooth needs Microspline or XD/XDR; an 11T minimum fits HG.
Key data
What it fits
The cassette must match your freehub and your chain. A 12-speed cassette won't fit an 11-speed system without a new chain, shifter and sometimes freehub. Across brands, a Shimano 12-speed cassette (Microspline) and a SRAM 12-speed (XD) use different freehubs and don't interchange.
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Frequently asked questions
What does 10-52T mean?
The smallest cog has 10 teeth and the largest 52. That's the range: the bigger the difference, the easier to climb and the more it demands from the derailleur.
Can I fit a wider-range cassette?
Yes, if your derailleur has enough capacity and max cog, and your freehub allows it. A 10T forces Microspline or XD/XDR.
Are cassette and freewheel the same?
No. A freewheel threads directly onto the wheel (old system); a cassette mounts on the freehub body, which carries the freewheel mechanism.