Why do keycaps matter? Well, think about it. You spend hours on your keyboard every day. And the part that you look at, and type on, is the keycaps. This means they are an important part of the keyboard that you should pay attention to. However, there are many nuances about keycaps that you should know about to make an informed purchase. Here's my guide.
Profiles
There are many different profiles, which range from flat and cubic to tall and curved. Each of these profiles is meant for to produce a different typing experience and a different sound. Very tall keycaps, with much hollow space inside, will produce a deeper thock while shorter keycaps will produce a light tap. There are quite a few different profiles, but the diagram shows the most popular five (mix 1.0 is a custom).
As you can see, each set's keycaps are slightly different depending on the row, which allows the keyboard to be more ergonomic. In commercial boards, the keycaps are almost always Cherry profile as the most people find it easy to type on. However, if you wanted a more uniform surface, you could try some more extreme keycaps such as the SA profile. The SA profile, as you can see, has sharply sloped keycaps which create a more flowing face. SA keycaps also make the board sound deeper because of their tremendous height. A wrist rest is definitely recommended since these keycaps are so tall.
Another shape of keycap is the artisan keycap. These are not meant for typing, but rather to add a little more personality to your board. They can range from lego figures to dinosaurs, and often are quite expensive.
Material
ABS
This is the most common keycap material, and is a type of plastic. It's very smooth, so much so that running your fingernail across it creates almost no noise. One of its biggest advantages is that it is inexpensive but decent quality, and that's why almost all stock keycaps are ABS. Unfortunately, ABS is prone to shine over time, and is also a grease magnet. When people think of disgusting, greasy office keyboards, they're thinking of ABS keycaps. This is not so say that all ABS keycaps will look disgusting - when kept clean, they look fine. They are, however, not the most durable kind of keycap.
PBT
PBT is the second most common material used in keycaps. It is finely textured, masking grease nicely, and is also more durable than ABS. Nicer keyboards usually come with PBT keycaps, and if your keyboard doesn't, you can get a basic set of PBT keycaps for around $25. The one disadvantage of PBT keycaps is that they have a lower set of optimal temperature ranges, but this would only come into play if you were, say, trying to clean your keyboard in the dishwasher (don't ask).
Rubber
A very exotic type of keycap, the most common rubber keycaps are made by the popular keycap company Tai-Hao. These are quite durable, and feel good for gaming. Rubber keycaps always come in sets of about 10-24, and are only for the left part of the alpha keys, first four numbers, and the arrow keys. They are mostly used for gaming keyboards, since it would feel weird to have two different types of keycaps in one board for typing. I think they have potential for being a typing keycap, but not too many others do and that is why they are only manufactured for certain keys. An important thing to note is that rubber keycaps look terrible with RGB, as the light is dampened by the keycap, but still allowed to shine through. The result of this is the upper half of the keycap lit, and the bottom half not, which is a tacky look in my opinion.
Pudding
These technically aren't a material, but I'm putting them here anyway. They're made of PBT on the top, and an opaque white plastic on the bottom. They are meant for maximum RGB diffusion, letting light shine through the letter and most of the sides. These are a controversial keycap choice, as while they look great with RGB on, they 1. look a little strange with no RGB 2. some people think so much RGB is obnoxious. You can choose for yourself - there are many lovers as well as many haters!
POM
An uncommon material, these are most popular in "jelly" keycaps. They are quite similar to PBT, but with better thermal stability. They are also very strong and have good impact resistance.
Steel/Aluminum/Titanium
These keycaps fit on the same areas where rubber ones do, and are also meant mostly for gaming or accents. They are much stronger, but keycaps are usually never replaced because they are broken, they're replaced because they've either started shining or there is a better option aesthetically. These keycaps are pretty aggressive-looking, and will only fit with some gaming builds. Additionally, with steel ones, they are so much heavier than some plastics, they will change the feel of those keys hugely if you have light switches such as speed silvers.
Layouts
There are quite a few different layouts, which are popular in different countries. The most popular in the USA is called the ANSI layout. We'll have pictures of the ansi layout, as well as some other international ones. Keep in mind that these layouts mostly only apply to the alpha and modifier keys. Additionally, the placement of the alt, fn, ctrl, and windows (or cmd), varies wildly from keyboard to keyboard, and layouts do not depend on these keys.
The ANSI and UK layout are most likely the only two layouts you will ever encounter, and the Dvorak layout, although more ergonomic, is seldom used due to the fact that it destroys muscle memory from any other layout.