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.
They come in a pack of 20, they're pretty handy so you'll want to keep them around.
You can find an EAGLE cad package for these in switch-omron:B3F-10XX
Example 6mm tactile switch datasheet for dimension information
Product Dimensions: 6.0mm x 6.0mm x 5.0mm / 0.2" x 0.2" x 0.2"