Resistor "Pull-ups" and "Pull-downs" serve many purposes in electronics. A pull-up is a resistor with one end wired to a positive power rail. A pull-down has one end wired to ground or a negative rail. Here is a basic introduction with schematics. A wikipedia entry covers this but is full of arcana like ttl logic gates.

This kit for the 2008 class included materials for scratch assembly of sensors, a CUI board running uOSC and a few tools such as scissors.