Pillar keycap, x-leg keycap and scissor switch are 3 main keycaps for a membrane keyboard.

Pillar keycap

This keycap has a pillar shape stand for a keycap. It is the most commonly used keycap for a membrane keyboard. It is easy to mold and made, but usually used in keyboard with high profile.

Disadvantage:

1, it can not be used to make low profile keyboard

2, users feel different when type on the edge of keytop instead of on the center.

X-leg keycap

The pressure on keycap can be sustained more balanced for a x keycap. Pressure is hold in 4 directions, not just one direction. X keycap is also in low profile, thus can reduce the size of keyboard. So, the x keycap can often be seen in some thin or slim keyboards.

X keycap is in fact a variation of scissor switch keyboard. It is used as a substitute for scissor switch in some thin keyboards because its low cost.

X keycap is also used in backlit keyboard because it does not block too much light. If light is block beneath the keycap, the illuminated letter will have shadow and not easy to see clearly.

Scissor switch keycap

Scissor switch is most widely used in notebook computer because of its low profile and perfect stroke. Perfect stroke is another name for scissor switch (or x-plunger, another name for scissor switch). Naming “perfect stroke” is because scissor switch has much better typing feeling for pillar keycap or x keycap.

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 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 commonly found in desktop dome keyboards).

Some content is extracted from wikipedia.org