Historically mechanical keys were used for keyboards, however, that trend changed to reduce costs of production and for many years it was challenging to find a good mechanical keyboard.
However, within the last few years, mechanical keyboards have become wildly popular. It's not uncommon to find a high-end keyboard manufactures touting the features of their specific key mechanism.
There are two parts to a mechanical key. The bottom portion with houses the physical switch and the electronic contacts, and the keycap which attaches to the key and what your finger makes contact with when you press a kit.
A variety of keys are available and each boasts it's own tactile feel as well as unique sound. Keycaps are also available in all sorts of colors, shapes, and graphics — you can even 3D print your own.