simple power socket question

[g8or]

what is the difference between two and three prong outlets/plugs?

[enigma]

The third prong is a ground plug. That way, if the device you're using shorts out, the current will flow through the third prong to ground instead of flowing through you to ground.

[Bystander]

It also furnishes a "polarized" connection of whatever device to the "hot" and neutral leads from the power source --- it's considered good form to have switches, fuses, and other interrupting controls in the hot line --- 2 prong plugs leave you a 50:50 chance of misconnecting the device, not that it won't work, but under adverse circumstances, you'd rather the fuse be on the hot line. You'll also find 2 prong plugs with wide and narrow blades to "fix" a preferred orientation for insertion in sockets, the wide blade being neutral and the narrow hot --- quite a few outlet plate covers demonstrate evidence of the wide blade being forced into the narrow "hot" side --- and some occasional indications of even more exciting results, such as scorch marks, spatter from molten metal.

Check the NEMA standards for further details.