EdgeForm[g] is a three-dimensional graphics directive which specifies that edges of polygons are to be drawn using the graphics directive or list of graphics directives g.
EdgeForm[ ] draws no edges of polygons.
The directives RGBColor, CMYKColor, GrayLevel, Hue and Thickness can be used in EdgeForm.
EdgeForm does not affect the rendering of Line objects.
No lines are ever drawn when an edge is formed by one polygon intersecting another.
See The Mathematica Book: Section 2.9.9.
See also: FaceForm, Line.