notes
When reading the value of a switch with Arduino, its useful to set the INPUT_PULLUP mode so you don’t need to use an external pullup resistor. By the way, the internal pull-up resistor is 20K ohms.
See this tutorial:
http://arduino.cc/en/Tutorial/InputPullupSerial
If its not clear what a pull-up resistor does, read the links on this forum post.
http://arduino.cc/forum/index.php/topic,22274.0.html
Everybody has their own way to explain it. I would say that when the switch is open, the pull-up resistor weakly pulls the logic value high, so you don’t get random signal readings.