[https://en.wikipedia.org/wiki/Coupling Couplers] transmit force; an example is connecting a motor to a gearbox or a gearbox to an axle or an axle to a wheel.  Each device needs a way to connect to the other.  The rotating part of a motor is the shaft and different motors have shafts of different shapes and sizes; to connect something to a motor, you'll need the appropriate coupler.

There are many [http://www.andymark.com/Hubs-s/215.htm off-the-shelf hubs] that are designed to connect rotating things.

For lighter loads, you can make 3D-printed couplers.  For example, the Denso window motor can be coupled using:
  - to a 1/2" keyed shaft using [https://www.thingiverse.com/thing:388315 this coupler]
  - to an off-the-shelf hub using [https://www.thingiverse.com/thing:1464800 this coupler]