A mechanical keyboard uses actual, physical switches underneath the keys to determine when the user has pushed a key. Press a key, and you press its switch down. Press the switch down, and the keyboard sends a signal to the PC telling it that you pressed that key.
At first, this design doesn’t sound so remarkable. After all, you already have a keyboard, and you can tell when you’ve pressed a key: You push one down, and a letter pops up on the screen. Take a second, however, to think about how you know you’ve pressed a key–it’s probably because you’ve pushed the key down as far as it will go, only after which do you see something happen on your PC.
Most keyboards are composed of a set of three plastic membranes, with rubber dome-shaped switches underneath each key. Press a key, and the rubber switch pushes through a hole in the middle membrane to connect the top and bottom membranes, which creates an electrical circuit that causes the keyboard to send the input to your PC. This keyboard design is inexpensive and spill-resistant, but it doesn’t give you as much tactile or audible feedback when you press a key, which can change the way you type.
Mechanical keyboards last longer: Mechanical switches are certified to last longer than rubber-dome switches pretty much across the board, regardless of the manufacturer. Even if a mechanical keyboard costs ten times what a cheap dome-switch keyboard costs, the mechanical one should last long enough to make the investment worthwhile–unless you spill your drink on it, that is.
Mechanical keyboards are heavy, obviously, mechanical keyboards aren’t designed for portability. Mechanical keyboard basically doesn’t move from your desk unless you pick it up. This may be a nice thing–you may hate it when your keyboard constantly slides around.
Mechanical keyboards (might) make you type differently: Some mechanical keyboard switches are designed to give you an audible “click” at the point where each button has actuated. Makes your typing more lightly, more easily, and maybe just a little bit more quickly–after all, the shorter the distance that each key travels, the less work for your fingers in the same amount of time.