In one category, we place forces which work in reverse. In other words, these forces do both positive and negative work on objects, both increasing and decreasing kinetic energy. We'll call these conservative forces .
Gravity is a conservative force.
Objects launched upward near the surface of the Earth slow down for awhile, losing kinetic energy but then begin to fall toward the Earth, speeding up and gaining kinetic energy. The force exerted by a spring is also conservative. In the second category, which we'll label non-conservative , we place forces which can only do negative work. Non-conservative forces can only reduce kinetic energies of objects. Friction is a non-conservative force.