Removing stains from concrete
There are lots of concrete cleaning products on the market, but I have had good luck with a 1 gallon solution of 1/4 bleach, 1 cup TSP and 3/4 water. Assuming you are talking about a driveway or garage floor, I would pressure wash the area first to get off the surface crud, then lay on some of the solution and let it set 5 mins or so.  Scrub it down with a stiff broom and power wash again.  Repeat this as necessary.
As a last resort, you can also use some muriatic acid, but test a small area first to be sure it doesn't effect the color too much.