Next to the cross the urn is very common in cemeteries. The urn symbolizes immortality. If there is a flame in it then it is symbol undying remembrance. If a wreath is on it then it is sign of mourning.
I have most commonly came across the urn draped. I grab this quote from about.com:
The cloth draping the urn symbolically guarded the ashes. The shroud-draped urn is believed by some to mean that the soul has departed the shrouded body for its trip to heaven. Others say that the drape signifies the last partition between life and death.