Retention Pond vs. Detention Pond

A retention pond is designed to hold a specific amount of water indefinitely. Usually the pond is designed to have drainage leading to another location when the water level gets above the pond capacity, but still maintains a certain capacity.

A detention pond is a low lying area that is designed to temporarily hold a set amount of water while slowly draining to another location. They are more or less around for flood control when large amounts of rain could cause flash flooding if not dealt with properly. For example, in my housing plat, we have a detention pond that all of the drainage from our streets run into. Normally it is a grassy field with a couple of concrete culverts running towards a drainage pipe. During the last couple of weeks with all the rain here in Ohio, all of our street drainage has run off into this area and filled it to near capacity
last week. As of today, you can see the water level nearly gone due to the planned drainage and evaporation.

http://www.newton.dep.anl.gov/askasci/eng99/eng99219.htm