A mechanical keyboard is a keyboard with a switch beneath each key. This is different from most keyboards, which have three membranes beneath each letter. Depressing the key pushes aside the middle membrane, which allows the top and bottom membranes to touch and send feedback to the computer.
In addition to being louder than most other keyboards, mechanical keyboards are heavier and more durable. The internal switches of mechanical keyboards make them more vulnerable to liquid than other keyboards. Most mechanical keyboards are older models from the late 1980s and early 1990s, though newer mechanical keyboards can still be purchased today.