Small planes tend to have keys, but not all.
The bigger the plane the less likely a key is needed and once you get into jets
, I can't think of any that need keys for engine starts.
The starting and securing of engines with no keys comes down to switches and knobs.
Likewise with exterior doors, on smaller planes including private end executive jets, you can find keyed exterior doors and luggage compartments.
With transport jets at airlines you won't find keys.
To secure those doors the jet bridge is pulled away from the airplane so there is no physical access to the boarding door.
aviation.stackexchange.com/users/258/casey
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.
aviation.stackexchange.com/users/2012/Dan Wiebe
Private aircraft tend to be parked at airports with no effective security, and owned by individuals who expect to start and lock it like a car.
I've seen many, and they all had ignition keys.
Small aircraft (mainly single-engine) are also more-or-less interchangeable - if you can fly a Cessna you can fly a Beechcraft without any real issues so theft is a genuine concern.
Commercial aircraft are typically parked at large airports with 24-hour security, these days likely armed.
And they are not interchangeable.
Put a 747 captain with 20 years experience in Boeings into an Airbus 320 and he won't be able to find the light switch, never mind fly it away.
aviation.stackexchange.com/users/1632/paul
Besides ignition keys, small airplanes (single engine land) can be equipped with throttle locks.
These metal devices fit over the throttle lever or vernier control and are secured with either a key or a numeric code.
I've frequently seen this kind of lock on rental aircraft.
Here's an example :
aviation.stackexchange.com/users/577/Philipp
C-160 TRANSALL has a door lock.
As a military cargo aircraft sometimes lands at remote airfields (or not even airfields), this is needed to "secure" the aircraft.
aviation.stackexchange.com/users/84/Simon
Heavy jet aircraft do not have keys.
You can enter the cockpit door which do not have locks, start up the APU - a small jet engine in the tail - to give you power and air.
Then start the main engines all of which just have toggle or push button switches.
It would be very easy to steal if you knew the systems.
As an aircraft mechanic for 40 years I feel there is a severe threat for this to happen.
I invented a small tool that would keep this from happening but no one is interested as of yet.
There is a pilot who attempted to steal a commerial aircraft on 7-17-2012.
I have contacted the TSA, FAA, Professional Pilots ass.
with my concerns and the answer to the issue.
I guess it will just have to happen before anyone listens!
aviation.stackexchange.com/users/3959/Jetmech
I can only speak to types I have experience with, so: Light GA singles typically have ignition keys.
Light GA twins typically do not.
When you get to the heavy stuff, I have no idea. :-)
aviation.stackexchange.com/users/49/Brian Knoblauch
I think the answer is: it depends.
The Citabria has two toggle switches, one for each magneto, and a push button starter.
Pilots who have never started one may have trouble because the accepted practice is to put the stick in the crook of your right elbow to keep the elevator back and the tail wheel on the ground, and your right hand on the throttle, which is on the left wall.
After flicking on the mags with your left hand high on the right wall, you press the starter button with your left hand.
It requires some practice!
But I think it might confuse a Cessna driver, or at least make him nose over.
Here's a picture of the interior of the LSA version of the Citabria:
My Mooney has a fancy high-security lock.
But even if you have the key, the 270HP engine, turbocharger, and laminar flow wings, not to mention the constant speed prop, cowl flaps, and high V-speeds will prevent even a 777-typed ATP from having a successful solo first flight.
aviation.stackexchange.com/users/3504/rbp
Some business jets like the Gulfstream for instance have a key to lock your passenger door from the outside, for instance if your aircraft is in night halt or you're gonna leave the aircraft parked for a day or so...
it's mainly for the security of the aircraft.
Most commercially used aircraft like Airbuses, have a CDSS (Cockpit Door Surveillance System), which allows the flight crew to see who is trying to access the cockpit door from the cabin.
Most of the time, a cockpit door with CDSS has a unique password to be entered on the touchpad of the cockpit door (at the entrance to the cockpit from the cabin) to get access to the flight crew compartment.
These passwords are normally known to the cabin crew and flight crews only.
Commercial aircraft normally do not have a door key to lock it from the outside as its security is monitored by airport security staff.
aviation.stackexchange.com/users/5543/Nikhil Kumar M
One thing I've always wondered is: do airplanes have keys that you put in the ignition to start?  And on the outside doors? Otherwise how are they secured and started?  I'm interested to know about a broad range of aircraft; from GA, all the way to airliners.
Do planes have keys?
aviation.stackexchange.com
<cockpit><security>
aviation.stackexchange.com/questions/3015
aviation.stackexchange.com/users/97/Danny Beckett
It appears that small planes have keys both for the ignition and for doors while large commercial airliners do not have any keys and are started with switches. It depends. Small planes usually have keys, although this is not always the case. Often a small plane will have only one door that can be locked from the outside - all the others are locked from the inside. Small planes may also have throttle locks. Specific examples of planes with keys include the C-160 TRANSALL, Light GA singles, the Mooney. The Citabria has two toggle switches and a push button starter. If a plane has no keys, the engines are started and secured using switches and knobs. Large commercial aircraft do not have keys. Most have a cockpit door surveillance system, which the crew use to see who is attempting to access the cockpit. Private aircraft do not usually have effective security when parked at airports and their owners like to be able to lock them with a key. For some business jets, the passenger door can be locked from the outside. 
Small planes usually have keys, although this is not always the case. Often a small plane will have only one door that can be locked from the outside - all the others are locked from the inside. Small planes may also have throttle locks. If a plane has no keys, the engines are started and secured using switches and knobs. Large commercial aircraft do not have keys. Most have a cockpit door surveillance system, which the crew use to see who is attempting to access the cockpit. Private aircraft do not usually have effective security when parked at airports and their owners like to be able to lock them with a key. For some business jets, the passenger door can be locked from the outside. Specific examples of planes with keys include the C-160 TRANSALL, Light GA singles, the Mooney. The Citabria has two toggle switches and a push button starter. It depends.