It’s a grounding wire plug.
A grounding wire is a safety measure to prevent a house fire or an electric shock if there is excess electricity in the wiring system. Ground wires run parallel to other wires in order to safely discharge excess electricity into the ground.
https://us-electric.com/what-is-a-ground-wire/
For electrical plugs, the ones that have 2 don’t feature a grounding wire. Many appliances don’t require it, and don’t pose any type of threat. But those that do have the 3rd wire.
The round prong is round just because the engineer that made it decided on that shape. No special reason. The 2 flat prongs are designed to be compatible with the older 2 prong type.
There are other prong sizes and shapes, but you won’t often use those types at home. The different sizes and shapes indicate different voltages, amperages, and intended uses. And they’re all specifically designed to 1)not fit each other, and 2) not fit your household outlets.
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