[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.  [http://www.andymark.com/Gears-s/85.htm Gears] and [https://www.andymark.com/Search-s/545.htm?Search=sprockets&Submit= sprockets] are also commonly mounted to rotating shafts and used to transmit force (via other gears or a [http://www.micromark.com/Web%20Store%20Images/84791_R-1.jpg chain]).  In all cases, the goal is to get the force generated by a motor to the mechanism that should be moving (wheels, arms, etc.).

For lighter loads, you can make 3D-printed couplers; you can find all sorts of useful pre-designed parts by searching for FRC on thingverse.com  
- 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]
- The Denso throttle motor can be coupled using:
  - a [https://www.thingiverse.com/thing:2016224 custom printed gearbox]