What is the difference between brushless Inrunner and outrunner motor?

Outrunner: The permanent magnets on the outrunner are placed on the rotor and the rotor spins on the outside case. On the inside of the motor are the stator windings which do not rotate, they are fixed in position.

For example: 

Outrunner Black Sealed Sensored Motors

Outrunner Open Cover Sensored Motors

Outrunner Sensorless Motors

Inrunner: On the outer side of the motor is the case. The case in this situation does not rotate and is fixed. The stator windings are placed on the inside face of the case. When you spin the motor shaft of an inrunner, you are spinning the rotor which also contains the permanent magnets much like the outrunner. The difference of course being that they are now at the center of the motor. 

For example:

Inrunner Waterproof/Watercooled Sensored Motors

Inrunner Sensorless Motors

Performance Differences

For simplicity, let’s loosely consider motors of equal size and weight in order to compare the possible performance differences.