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 above 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.
Along with the pyroelectic sensor is a bunch of supporting circuitry, resistors and capacitors.
For many basic projects or products that need to detect when a person has left or entered the area, or has approached, PIR sensors are great.
They are low power and low cost, pretty rugged, have a wide lens range, and are easy to interface with. Note that PIRs won't tell you how many people are around or how close they are to the sensor, the lens is often fixed to a certain sweep and distance (although it can be hacked somewhere) and they are also sometimes set off by housepets. Experimentation is key!
How does it work?
PIR sensors are more complicated than many of the other sensors explained in these tutorials (like photocells, FSRs and tilt switches) because there are multiple variables that affect the sensors input and output.