"Stamped" is just like it sounds -- the cog is stamped out of sheet metal. Better cogs are forged and machined. The cog on the right below is stamped; the other is forged. If you look at the center section on the stamped cog, you can see how the sheet metal curves down to form the flange for the threads. On the other cog this is a sharp, machined transition. It's difficult to see in the picture, but the forged cog also has tool marks from the machining process.