extract extracts faces from a mesh, separate separates pieces based on the border edges
so, if you have a cube, you select a face and go extract the you've taken that face off from the cube and now you have two distinct pieces of geometry
if you did separate on that same face nothing will happen
essentially separate is the exact opposite of combine and extract the opposite of append polygon
that's a "Ch" pronounced as a "K"
Computer skills I should have:
Maya, XSI, Photoshop, AfterEffects, Motion, Illustrator, Flash, Swift3D