Both a retention pond and a detention pond hold storm water runoff but the difference between a retention pond and a detention pond is that the retention pond holds storm water longer than the detention pond does.
The purpose of a detention pond is to temporarily store stormwater runoff, thereby reducing the peak rate of runoff to a stream or storm sewer.
The detention pond also helps to prevent localized flooding and, if designed to do so, provide some water quality benefits and reduce streambank erosion downstream.
The purpose of a retention pond is to treat and store storm water runoff also.
A retention pond is a permanent pool or pond of standing water and eventually the water will empty into a receiving water body.
The retention pond helps to prevent flooding as well due to storm water that could overflow other things and then flood homes and businesses.
It also helps keep bad things out of the water that can get into the water during the storm.
So then the water in the retention pond is stored, treated and then sent back into a lake or other place that is close to the retention pond.