Climb milling or down-milling is when the work is moved in the same direction as the rotation of the cutter. Full engagement of the tooth is instantaneous. The sliding action of conventional milling is eliminated resulting in a better surface finish and longer tool life.
Conventional or up-milling is when the work is fed into the rotation of the cutter. The chips are at a minimun thickness at the start of the cut and are so light that the tool has a tendency to slide over the work. This sliding, followed by a sudden breakthrough as the tool completes the cut, causes a rough surface finish.