IR is light energy at a frequency that is invisible because it is below a red color. It can be sent from an IR Light emitting Diode (LED), from the sun or from anything that is hot.
An IR receiver can be an IR diode (it needs an amplifier), an IR Photo-transistor or an IR receiver integrated circuit that is usually used.
To avoid interference from the sun or from anything that is hot then the IR is pulsed at a frequency around 38kHz and an IR receiver integrated circuit is most sensitive at that frequency. Most IR receiver ICs have an automatic-gain-control that reduces the sensitivity when there is continuous IR pulsing which is produced by compact fluorescent light bulbs and fluorescent tube lights.