Normal 'membrane' keyboards have a sheet of rubber domes under the keys.
Mechanical keyboards have a mechanical key switch under every key instead.
These switches are much more precise than rubber domes, and much more satisfying to type on.
Mechanical key switches are composed of a housing, a stem, and a spring.
The spring controls the actuation force, and the stem
Depending on the stem design, the switch can be linear (smooth), tactile (bump), or clicky (click and bump).
Personally, I prefer tactile switches with around a 67g bottom out force.
I might be a bit obsessed with mechanical keyboards.
I currently own 6 mechanical keyboards, 6 custom keycap sets, and around 15 artisans (see Artisans).
I've also made and sold artisans of my own (see Resin).