Friction

[palaniyapan]

How is friction is reduced by metal ball bearings?

[Clanad]

The curved surface of the "ball" has the smallest contact with its "race", thereby producing the smallest amount of resistance and friction...

[gazzawazza]

Friction is reduced because instead of two surfaces rubbing together they roll to allow the movement.

[Space]

gazzawazza's answer is correct, Clanad's isn't.

[Clanad]

And how is that Space?

[Space]

Ok, I'll try to give a more constructive and less patronising answer...

Friction between solids doesnt depend much on how big the contact surface is. It mostly depends on the materials used, the roughness of the surfaces, and the forces applied.
http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html#plo

The basic idea behind ball bearings is that solids roll against each other instead of sliding, and that "rolling friction" is much smaller than the friction that happens when solids slide.
http://science.howstuffworks.com/bearing1.htm
Also, a variant of ball bearings uses cylinders instead of balls, so they have a much bigger contact, but that doesnt give them more friction, it just makes them capable to handle heavier loads.

[Thunderchild]

correct