[seanwitte]

A thermistor is a component whose resistance varies with it's temperature. If you're using a microcontroller you can measure the temperature using an analog input. To connect the thermistor to the controller you connect one end to +5V and the other end to a 220 ohm resistor. The second lead of the resistor goes to ground. The analog input gets connected to the node between the resistor and the thermistor. The circuit is called a voltage divider. There is a large selection of parts at digikey.com. Radio shack might sell them as well. Excuse the ascii diagram, but you get the idea.
[code]
+5V
o
|
[ ] thermistor
|
o-----o to analog input
|
<
> 220 ohm resistor
<
|
o
Ground