As a private pilot, single-engine land, I've flown maybe fifty different light planes--none with any more than four seats or 210HP--and all of them have had ignition keys. I haven't been in the cockpit of anything bigger.
Generally the keys have five positions: OFF, MAG 1, MAG 2, BOTH, and START. You insert the key at the OFF position, turn it all the way to START and hold it there until the engine catches, and then release it back to BOTH. MAG 1 and MAG 2 (sometimes LEFT and RIGHT) are for the runup check just before takeoff to make sure both your magnetos are working.
Usually the same key that fits in the ignition switch fits in the door lock. It's pretty common for a small plane to have only one door that's unlockable from the outside: you lock all the other doors from the inside, then climb out and lock the last door.
One thing I've noticed, though, is that keys and locks for small planes don't appear to be serious--at least, not the same level of serious that you see on cars and buildings. They're more serious than briefcase keys or computer-case keys, but they look about as pickable as a cheap padlock...and while I've never looked, I wouldn't be surprised if you could reach under the edge of the instrument panel, pull wires with spade lugs off the back of the ignition switch, and touch them together yourself.