Irrigation Rain Sensors are devices that are connected to the sprinkler systems controller either via hard wiring the rain sensor to the controller or through the use of a wireless transmitter and receiver.  Rain sensors, as their name would imply, detect the presence of rainfall and will disable the irrigation controller from operating during periods of wet weather.  Available in both normally open and normally closed circuit models, these small devices are configurable  to whatever your situation calls for.  Most models include mounting brackets for installing on the side of a gutter or the eave of a house.  They are generally adjustable to the amount of rain required to disable the irrigation system controller.  Most utilize a small disk, which once wet will enlarge and break the circuit to the controller.  When the weather clears and the disk has a chance to dry out, it again shrinks back down and completes the circuit once again.  These small devices are well worth their cost by saving on unneccessary waterings during periods of wet weather.