below are definitions of each. think about it this way... a ball bearing is just a small, round ,metal ball. It's usually installed into some sort of assembly that's made to go around in circles....
a small metal ball about half a centimeter high used to make things rotate for example:the pedals on your bike move around with the help of ball bearings
Ball & Roller Bearings both belongs to the category of radial contact bearings. In case of ball bearings spherical balls are used however in case of roller bearings cylindrical rollers are used....