A conditioner does just that....it conditions the leather by re-hydrating it with emulsions and natural oils to keep it from drying out and cracking. Keeping it supple makes it look better and last longer.
A cleaner, well, cleans. Good ones are ph balanced and have no harmful solvents or harsh detergents.
I have tried Meguiars, Mothers, UPP, and Hi-Temps Leather Cleaner/Conditioner and Hi Temps is by far the best. The UPP I actually hated, and the hi temp will clean through stains that nothing else will, just use some elbow grease.