Mechanical keyboard switches come in three broad families: linear, tactile, and clicky. Within each family there are dozens of specific models with subtle differences, but the family decision is the one that matters. Get this right and the rest is fine-tuning.
Linear
Linear switches feel smooth all the way down — no bump, no click, just a steady push to the bottom. Most gaming-oriented switches are linear, because there is no tactile feedback to mask quick double-presses. Many typists also prefer them once they get used to the lack of feedback, because the typing motion feels more fluid.
The downside is that you can press a key without realising it. People moving from rubber-dome keyboards often produce typos for a few weeks because they expect the keyboard to push back and it does not. After a month most people adapt.
Tactile
Tactile switches have a small bump partway through the press — you feel it before the key registers. The bump tells you the keystroke has succeeded without needing to push the key all the way down. Many people find this the most comfortable family for long typing sessions.
The size and position of the bump varies widely. Some switches have a sharp pronounced bump near the top of the press. Others have a softer rounded bump in the middle. If possible, try a switch tester before committing — it is hard to predict from a description what you will like.
Clicky
Clicky switches have a tactile bump and a literal click sound at the same point. The click is loud — significantly louder than other switch types — and most people who try them love them at home and never use them in an office. If you share a workspace, clicky switches will make you unpopular within a week.
The classic clicky switch is the Cherry MX Blue, but the modern alternatives (like the Kailh Box White) have a sharper, more defined click that many people prefer.
How to choose
If you mostly game or want a smooth quiet typing feel: linear. If you mostly type and want feedback without the noise: tactile. If you work alone and love the sound: clicky. If you have no idea, get a tactile switch — it is the most forgiving and most people end up liking it.
A switch tester (a small board with one of each switch type) costs about twenty euros and saves expensive mistakes. Worth doing before buying a full keyboard.