I have several keys (all the same cut) for my 1994 Toyota Camry - all will start the engine, but only one will unlock the door from the outside and none will open the trunk.

It has almost twenty year old tumblers. They are brass and wear out. Try moving the key up and down while turning.

Here's the basics of how they work. (http://en.wikipedia.org/wiki/Pin_tumbler_lock) --- I used to work on the Rochester Products line that cut the keys and tumblers. We cut the same sets for ten cars, changed one notch (number) and cut ten more sets. Theory was that the cars were randomly spread all over the country so having the same key wouldn't matter. --- When I was a bag boy at the supermarket, occasionally people would try their key in the wrong truck and it would open because of the slop in the keys. When the stuff in the trunk was different, they realized that it was not their car.