Tactile Button switch (6mm) x 20 pack
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.
Product Dimensions: 6.0mm x 6.0mm x 5.0mm