Mechanical keyboards are generally called mechanical, due to the switch having some sort of plastic or metal part in it which changes the feel, and contact method.
This, for example is a Cherry MX Red switch; it has a plastic red part, which compresses on a spring, and a metal part forms a contact which registers the keypress
![cherry mx red](../../I/static/images/2816b61be0146bc24a0d00588c66cff889f642f7aacd7368b99c7a8d2d645632.gif)
On the other hand, the average $20 keyboard has a membrane switch; instead of having a metal part which makes a contact, there is a silicon dome inside, which is compressed onto a circuit board to make the contact. There are high quality ones for up to $20 or more, while cheaper ones, less than $10 tend to be spongy, mainly if they've been worn, or badly manufactured.
![rubber dome](../../I/static/images/039e9d07492072fa8c1d7ac3170291fb2d03df82ce8cae61170e1b83c487f3ed.jpg)
1Membrane switch. – dirkt – 2016-11-07T20:55:37.300
1
They use a keyboard membrane which makes contact with the circut to create and send the input signal.
– DrZoo – 2016-11-07T21:02:31.247