An IR transmitter contains an LED that emits infrared light.  Thus the name.  The receiver contains either a photodiode or a phototransistor (usually the latter).  This component passes more or less current depending on the amount of IR light falling on it.

The LED is switched on and off in a coded sequence.

The IR Receiver is a type of Diode but different from the LED. It does not emit any light but detects the presence of light. This diode allows some current to pass through it after detecting light and amount of current flow is directly proportional to the amount of light detected and makes a pair with IR transmitter.