All bearings have the same purpose, to support loads generated by either weight or torque while allowing parts to rotate with minimal friction. Friction reduction is important for bearings as it increases efficiency and extends the life of the bearing. Bearings reduce friction by virtue of their shape, material they are made of, usage of a lubricant, or some combination of those three.
Bearing supports two types of load: radial and thrust. Radial load is perpendicular to the bearing and the centerline of rotation while thrust load is parallel to the rotating axis and the centerline of rotation. In many situations these load are combined, as with a differential pinion.