The two-stage snow thrower essentially has two main mechanical components working together to clear snow. The two components are where the expression "two-stage" comes from. The first stage/component is the 'auger' which is the spiral shaped steel located at the front of every two-stage snow thrower. The second stage/component is the 'impeller' which is a fan like disc located just behind the auger.

 

Here is a general picture of a 2-Stage unit.