From BEAM Robotics Wiki
Often simply called a "button" or "pushbutton", and typically having a surface that is flat or shaped to accommodate the human finger or hand, so as to be easily depressed or pushed.
The most common type of push-buttons is the Single Pole, Single Throw (SPST) momentary type. This means the switch has one set of contacts that will be open or closed depending on whether the switch actuator is being depressed or not, and that there is a spring that returns the actuator to its resting position when it is no longer being depressed.
If the contacts in a momentary push-button switch are “open” until the button is pushed, then it is a “normally-open” (NO) push-button or “push-to-make” switch. If the contacts are closed until the button is depressed it is a “normally closed” (NC) push-button or “push-to-break” switch.
Other configurations include but are not limited too:
There are also “push-on/push-off” or bistable type of push-button switches, in which the contacts are moved into and held in one position when the switch actuator is depressed once, and the opposite position when the actuator is depressed again. This would be analogous to throwing the lever of a toggle switch first one way and then the opposite way.
 Make-before-break, Break-before-make
Just a quick note about something that might be important when using a salvaged push-button switch.
There are two possible transient behaviors that the contacts of a push-button switch can exhibit when the actuator is pushed. In some designs, the new contact is made before the old contact is broken. This is known as “make-before-break”, and ensures that the moving contact never sees an open circuit (also referred to as a “shorting switch”). The alternative is “break-before-make”, where the old contact is broken before the new one is made. This ensures that the two fixed contacts are never shorted to each other.
“Make-before-break” is not uncommon so be careful when using salvaged switches. It would not be a good idea to use a “shorting switch” to select between to logic signals that might be in opposite states, or power supply lines that might be of the opposite polarity.
For more information see the Wikipedia Article: Push button