IR receivers are little diodes with a photocell that are tuned to detect to infrared light. They are usually used for remote control detection - every TV and DVD player has one of these in the front to listen for the IR signal from the clicker.
Inside the remote control is a matching IR LED, which emits IR pulses to tell the TV to turn on, off or change channels. IR light is not visible to the human eye, which means it takes a little more work to test a setup.
For IR receiver datasheet, please click here
For nice explanation of IR remote signal basics, click here