Answer:
Wheel is used to carry the bicycle, of course. In order to fulfill it's role, the wheel needs to be: True. Without any dens or wobbles.
The axle is attached to dropouts on the fork or the frame. The axle can attach using a: Quick release - a lever and skewer that pass through a hollow axle designed to allow for installation and removal of the wheel without any tools (found on most modern road bikes and some mountain bikes).
