Can anybody explain the difference between an inrunner & outrunner motor and how they compare to a standard 540 motor?

Here you go. 540 size (brushless) motors are inrunners. Outrunners are typicaly used on planes.

Stole this from http://www.hooked-on-rc-airplanes.co...rc-motors.html
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Inrunners vs Outrunners

There are two types of Brushless RC motors, inrunners and outrunners.

The permanent magnets of inrunner brushless motors are positioned on the inside of the electromagnets.

Inrunner brushless motors are set up very similar to the canned motor explained above, except the permanent magnets and electromagnets are in opposite positions.




An outrunner brushless motor has the permanent magnets on the outside of the electromagnets.

You can see in the picture to the right that the outer hub holding the permanent magnets has the output shaft attached in the center.



So which one is better?

The faster a motor spins, the more efficient it is. Inrunner motors turn very fast and are much more efficient than outrunner motors. Inrunner Brushless RC Motors require a speed reducing gearbox between the motor and propeller of your RC airplane.

For this reason, the output speed and torque of the propeller can easily be "tweaked" to facilitate different flying characteristics by using different size gears.

The downside of an inrunner is the added parts that can and do fail. The gears get stripped, and the gearbox shafts are easily bent. It can also be an obstacle when mounting the gearbox motor combination for your RC airplane neatly, especially under a cowling.

Outrunners


Low RPM's, high torque
Less efficient than inrunners
No gearbox required
Narrow prop selection
Silent

Inrunners


High RPM's, low torque
More efficient than outrunners
Require a gearbox
Wide prop selection
Noisy

Thanks HBRob, I was more than just a little bit confused. The confusion was because some shops have these motors listed in their 'car motors' category.