I'm glad@SaraBeaman asked me this question. I think a lot of us don't know the difference between the two. So, here's a card explaining the functionality of bobby pin and hairpin.
Bobby pin was invented during the 1899 and made popular in 1920s to secure the bob for flappers. Both the bob cut and bobby pin were invented by Robert (Bobby) Pinot. The style and tool were name after him.
Bobby pin: Secure your hair tightly while changing the shape.
Hairpin: Secure your hair loosely without changing the shape (i.e. curls)
As you can see bobby pins and hairpins has different size for specific purposes
This video goes straight to the point and teaches you how to use a hairpin!
The video above demonstrates how to use bobby pin to create a messy updo. No elastics needed!