Low battery, wiring at the battery, ground strap, starter relay, solenoid, starter motor, neutral safety switch, ignition switch. And the wiring to the starter.
Start by cleaning the battery terminals, make sure your battery is fully charged. Listen carefully (hood up and door open) when you turn the key--do you hear a faint click under the hood? If so, you know the ignition switch, neutral safety switch, and solenoid are all good. If not, you can bet it's one of these. Neutral safety switches break more often than any of the other parts.
If you've got that click and nothing else happens, it's probably the starter, bad connection at the battery, undercharged battery, or the grounding strap. If you turn the key with your headlights on, and the lights dim significantly--it's probably a bad starter.
To test your ground strap (and make a temporary repair), use your jumper cables to connect the negative (black) terminal on the battery to your engine block, and try it again.
Before you replace the starter, take it off and carry it to your auto parts store. Most stores will test a starter for free.
