A Mileage Run is when a perfectly sane person loses all sensibilities and pays to fly madly around the world in search of more redeemable miles to take free trips with.

A Status Run is the same, expect the primary benefit is to earn higher status to be treated better than others in many aspects of flying.

A Segment Run is not trying to earn miles, but rather the maximum number of take offs and landings on different flight numbers. Usually used to gain status.

I have been guilty of the first two. Never the third. And I never would. At least I don't think I would...