I work with polygons all the time in 3D Studio Max 6
To me the higher the poly the smoother the object...the lower the poly the ridged the object. If you limit the number of poly's on an object it will be a lot smoother (From a games point of view) but will be bricky as You see here>>
My Computer Forum Mascot has got High poly and medium Poly so thats how the face is smooth but the hat and eyes are just medium because they dont need to be high
basically, its a shape with straight edges.
in computer terms its what makes up a graphic (like pixels, although thats to do with your monitor, not graphics (i think...)).