mock-up exists out side the internet. it's not an unfinished web site.

a mock up is a conceptul drawing, a very early prototype if you will. think of a mock up as a non-tehcnical drawing - none of the numbers are exact. for example, you're laying out a room: you have some shapes and labels like "bed here, dresser here, door here", etc.

it's basic purpose is to take your raw ideas and put them down on paper (or screen if you can avoid printing something digital). think of them like blueprints, only without any of the details/calculations. mock ups can be as simple as a sketch on a ****tail knapkin.