Difference between Geographic and Projected coordinate systems?

What is the difference between a geographic coordinate system and a projected coordinate system?

Answer

Geographic Coordinate Systems:

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/What_are_geographic_coordinate_systems/003r00000006000000/

Projected Coordinate Systems:

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/What_are_projected_coordinate_systems/003r0000000p000000/