>>1543555The differences are by brand - usually they're not cross-compatible because each works slightly different, "number of speeds" - how many rear wheel gears you have, quality control, and how light they are. The top end is electronic shifting rather than cables.
If you look at older parts the "tiers" become less clear because the number of gears increased over the decades.
You can use the same chain for anything 5, 6, 7, or 8 speed. 9/10/11 speed each have their own (narrower) chains. At 7 speed and below the gears are a self-contained unit called a "freewheel" with a ratchet inside it that goes onto a threaded hub. Above that they are a "cassette" that is slid onto a hub with the ratchet integrated into it (called a freehub). Generally (but not always) 8, 9, 10, 11 speed each require a different style hub.
A few 8/9sp freewheels exist, but for ebikes which have a stronger axle.
There are also "IGH", internal gear hubs. These have a gearbox inside. They're usually heavier and have a reputation (deserved or not) for being more reliable. Mostly found on cruisers and old bikes.
Shifters are usually divided between "mountain" and "road", with IGH having their
own special shifters.
Mountain shifters are trigger shifters that work with a lever you push with your thumb and/or finger.
Road shifters are either a pair of levers that attach to the downtube or stem, or they're integrated into the road bike's brakes with a smaller lever behind the brake lever that's moved sideways.
There are some other weird ones that are basically the road-type levers just stuck in a different location in an attempt at convenience.
And twist shifters, which suck.
Most shifters only work with 1 set of speeds.
The exceptions are that some 7/8 sp shifters can be limited to work with less gears. And "friction" shifting, which has no clicks and just freely moves the rear derailleur around. Very old levers are always friction and it's a selectable mode on some mid-to-low end ones now
