Detectors act much like vanilla detector rails, but are more versatile. When placed adjacent to a rail, it will trigger a redstone signal when a minecart passes. Unlike the vanilla detector rail, detectors can be placed under or even next to a rail. This allows a detector to function on more than one track if placed appropriately. However, they only supply redstone current on the face marked with a red light.
Detectors come in several different varieties that can either trigger on the specific type of cart that passes, or any cart.