PIR sensors allow you to sense motion, almost always used to detect whether a human has moved in or out of the sensors range. They are small, inexpensive, low-power, easy to use and don't wear out. For that reason they are commonly found in appliances and gadgets used in homes or businesses. They are often referred to as PIR, "Passive Infrared", "Pyroelectric", or "IR motion" sensors.
PIRs are basically made of a pyroelectric sensor (which
you can see below as the round metal can with a rectangular crystal in
the center), which can detect levels of infrared radiation. Everything
emits some low level radiation, and the hotter something is, the more
radiation is emitted. The sensor in a motion detector is actually split
in two halves. The reason for that is that we are looking to detect
motion (change) not average IR levels. The two halves are wired up so
that they cancel each other out. If one half sees more or less IR
radiation than the other, the output will swing high or low.