What is the difference between a Retention pond and a Detention pond?

Detention
Generally, detention ponds provide only flood control measures and are known as dry ponds.  Detention ponds help control the rate of flow by using a control device that maintains the pre-development rate of flow. The volume of the detention pond is calculated by comparing the pre- and post- development runoff volumes.  The difference is the detention volume. Usually, the controlled device is placed at the entrance to the outlet pipe to control the rate of flow to the pre-development rate. The pond is intended to drain the storm water within a period of time to make the volume available for the next storm event.   As shown in the diagram below, the outlet pipe (or control device) is placed at the bottom elevation of the detention volume to allow the pond to drain dry.

Retention
Unlike dry detention ponds, retention ponds hold a permanent pool of water and are referred to as wet ponds.   Usually a retention pond is constructed because of a high groundwater table (in other words, the groundwater is near the surface of the earth). The bottom of the pond is excavated below the water table elevation to establish a permanent pool.   The outlet of the pond is placed at or above the desired pool elevation. The volume of the permanent pool is set by a desired residence time to allow microbes and vegetation in the water to consume nutrients and to allow suspended pollutants to settle.   In general, retention ponds  require more area than a detention pond.  This is due to constraints in the allowable depth of water to maintain the vegetation on the pond.  The level of water in the pond is maintained by setting the outlet structure above the pond bottom at the groundwater elevation.