Little clicky switches are standard input "buttons" on electronic projects. These work best in a PCB but can be used on a solderless breadboard as shown in this tutorial. The pins are normally open (disconnected) and when the button is pressed they are momentarily closed.
These buttons are bigger than the 6mm ones they carry, and come in a pack of 15. Each button comes with a candy-colored round cap that snaps on. You get three of each color-- blue, grey, yellow, green, and red. The round plastic button tops affix easily to the tactile switch body.
Assembled button dimensions: 12mm x 12mm x 12mm