An Infrared Receiver (IR receiver for short) detects infrared light modulated at a certain frequency, commonly 38 kHz. Whenever this frequency of the IR spectrum is detected, the output pin of the IR receiver is asserted to low (leftmost pin of the IR receiver in the diagram below). When a regular TV remote is pointed to the IR receiver below, the red LED will light up. One of the reasons the IR signal is modulated at 38 kHz is so that the IR receiver can distinguish it from naturally occuring infrared light (e.g., the sun). Each button of a TV remote sends an unique on/off pattern that identifies the particular button. An oscilloscope or a logic analyzer are needed to visualize this pattern. The Arduino is once more only used as a power supply in this example and no sketch needs to be uploaded.
A good explanation on the functioning of an IR receiver is contained at this Adafruit tutorial. This web page provides a good explanation of the various encodings used for IR signals.
