The Decimate modifier is essentially a very simple form of polygon reduction. For this example, I'll start off with an icosphere.
First I'll make something that needs to be reduced, I'll add a Subsurf modifier and apply it. Now we have a sphere with way more vertices than is needed.
So now, I'll add a Decimate modifier, and not that you can control how much of the mesh is reduced by the Ratio slider, and the Face Count shows how many faces are now remaining in the mesh.
Note that you will only be able to observe the changes from the Ratio slider when in Object View [TAB].
The reduced mesh is shown to be almost the same as the original, though not as uniform.