Let us say we created a mesh object, but ended up with a lot more number of vertices than you think is the optimal number. To maintain the overall shape and still reduce the vertex count, decimate modifier can be used. The way to use it is explained below:
Add a Monkey object and change to edit mode. In the info editor header the number of vertices is shown as 507. Change to solid mode. Having the monkey object selected, go to Properties Editor and click on the Modifier icon(Spanner), then click on Add modifier button. In the drop down menu under "Generate" click on "Decimate". Under the "Collapse-Un-Subdivide-Planar" buttons, "Ratio 1.0000" is visible which is a slider control. Click LMB and drag to change the value from 1.0000 to say 0.700. Next click on Apply button. The number of vertices would have reduced to 360.
As you reduce the ratio value, the vertex count will reduce, but beyond a point the details of the mesh shape will tend to be lost.
