It's from railcraft I believe, it detects....sheep. If you have a sheep farm with sheep in it and used a embarking track to load up sheep to pass through a sheering station, the detector will detect if the ship needs sheering and pass a redstone signal to whatever sheering mechanism you have going. If the sheep doesn't have wool it will not pass the redstone signal.
edit-----
Found it.
Wiki is your friend!
Detector - Sheep
Triggers only on carts with Sheep in them with enough Wool to be sheared.
Has a GUI for selecting specific colors of wool
