Spotted spurge has a more upright growth habit while prostrate spurge hugs the ground and the stems root at the nodes.  Each leaf of both varieties contains a reddish/purple spot. 

You will commonly find prostrate spurge in the cracks of sidewalks and driveways.  It has a long tap root, and when it is young, it is easily pulled when the soil is moist. 

Life Cycle:  Annual broadleaf weed that reproduces by seed.  Prostrate spurge blooms from July to September.

History:  Prostrate spurge is native to the United States.  All spurge species are toxic if eaten, especially to cattle, and the milky white sap inside can cause skin irritation.  (from the University of Vermont Common Weeds of the Northeastern U.S.)