There are multiple types of pins and I am addressing the most common types.
1: Regular
2: Mushroom
3: Spool
4: Multi
5: Serrated
6: Waffer
Type into google images and examine how these pins look.
Regular pins are basic cylinders and don't catch on the plug as obnoxiously as mushroom or spool pins. Mushroom and spool pins are shaped as their name implies and catch instead of moving to the shear line. Serrated pins are extremely annoying in that they are cut with multiple lines "serrated" and are constantly catching as you try and turn the plug.