When you get to a certain level of gymnastics, you have to wear grips. You wear them because you have to jump to the high bar. You need to make sure that you have to stay on the high bar so you don't fall off.
Dowel grips are used by competitive gymnasts doing high level circling elements like giants. The dowel is a rod near the gymnast's fingers that helps her get a better grip on the bar. Beginner gymnasts don't need this and so they use a non-dowel simpler grip.
Beginner grips have Velcro straps to go around your wrists so it is easier to put on.
Some people put chalk on their grips to go around the bar when they're doing giants. (Giants are when you're on the high bar and swing in circles)