THE MIRROR TOOL ALLOWS YOU TO REFLECT A PART OR FEATURE OVER A PLANE TO DUPLICATE FEATURE
See DD02: Extrusion for explanations of New, Add, Remove, or Intersect
The PART MIRROR will create a solid model that is a mirror image of a selected part reflected over a plane
The FEATURE MIRROR will add a mirror image of a feature (Extrude, Revolve, etc) reflected over a plane