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Dome switch keyboards are a hybrid of flat-panel membrane and mechanical keyboards.
They bring two circuit board traces together under a rubber or silicone keypad using either metal "dome" switches or polyester formed domes.
The metal dome switches are formed pieces of stainless steel that, when compressed, give the user a crisp, positive tactile feedback.
These metal types of dome switches are very common, are usually reliable to over 5 million cycles, and can be plated in either nickel, silver or gold.
The rubber dome switches, most commonly referred to as polydomes, are formed polyester domes where the inside bubble is coated in graphite.
While polydomes are typically cheaper than metal domes, they lack the crisp snap of the metal domes, and usually have a lower life specification.
Polydomes are considered very quiet, but purists tend to find them "mushy" because the collapsing dome does not provide as much positive response as metal domes.
For either metal or polydomes, when a key is pressed, it collapses the dome, which connects the two circuit traces and completes the connection to enter the character.
The pattern on the PC board is often gold-plated.

A special case of the computer keyboard dome-switch is the scissor-switch.
The keys are attached to the keyboard via two plastic pieces that interlock in a "scissor"-like fashion, and snap to the keyboard and the key.
It still uses rubber domes, but a special plastic 'scissors' mechanism links the keycap to a plunger that depresses the rubber dome with a much shorter travel than the typical rubber dome keyboard.
Typically scissor-switch keyboards also employ 3-layer membranes as the electrical component of the switch. These stabilizing scissor-like devices extend the lifespan of the membrane to as much as 10 million keystrokes.
They also usually have a shorter total key travel distance (2 mm instead of 3.5 – 4 mm for standard dome-switch keyswitches).
This type of keyswitch is often found on the built-in keyboards on laptops and keyboards marketed as 'low-profile'.
These keyboards are generally quiet and the keys require little force to press. Scissor-switch keyboards are typically slightly more expensive.
They are harder to clean (due to the limited movement of the keys and their multiple attachment points) but also less likely to get debris in them as the gaps between the keys are often less (as there is no need for extra room to allow for the 'wiggle' in the key as you would find on a membrane keyboard).