With a mechanical keyboard, users can pull the key from each individual switch inside the mechanical keyboard. These key are called the keycap. There are usually two different kinds of keycaps, individual or keycaps sets (meaning a individual keycap or a set of keycaps with a theme.).

Here is a keyboard with lots of individual keycaps mixed together:

Here is different individual keys

Can you name all of them

Here is a keyboard with a set of keycaps

Just in case if you don't know this, but this is an EVA set keycaps

You can have image printed on to the keycap, usually there's two different kinds of profile (the size of the keycaps), and they are cherry ot OEM profiles. Both should fit most standard cherry mx switches mechanical keyboards. The three most commmon materials are ABS (Acrylonitrile Butadiene Styrene), PBT(polybutylene terephthalate) and POM(Polyoxymethylene), these are all made of out plastic, but different ones.

Here's how a set of ABS keycaps look like

How we know this is ABS? It's pretty simple, most PBT doesn't support backlit. Meaning the LED will not shine on the legend in the keycap.

There's different printing tech and one of the most common one is double-shot injection molding, this pretty much means that there's two molding instead of one. What that mean is that the legend on the key isn't print but rather having two different molds together. Yes, you can have PBT as one mold and POM as the other one. There are a lot of cool stuff like 3D printing a keycap. The coolest of all are artisan keycaps. Let say if everyone likes Hello Kitty (Not really!), someone will be like "Oh shot, I can totally make a keycap with Hello Kitty's icon on it and people will buy it". There are a lot of cool artisan keycaps and below are some examples.

What???

Yes, people would spend hundreds to buy one keycap.

I enjoy PBT keycaps, because the ABS is very smooth (Wait, what? You don't like smooth keys?). The reason is the smooth keys will have your hand sweat on top if you sweat a lot. Over time the key will started to have "oil" (Not really oil, but what it mean is that the layers in the key will started to peel off and the legend on the key will change, ever wonder why your X key on your keyboard disappear or changed shape?). PBT doesn't have this kind of problem, but it isn't a smooth as ABS keycap.

Here is PBT rainbow set

Here is PBT raindrop set