Sensor

A sensor is a device that measures a physical quantity and converts it into a 'signal' which can be read by an observer or by an instrument. For example, a mercury thermometer converts the measured temperature into the expansion and contraction of a liquid which can be read on a calibrated glass tube. Video cameras and a digital cameras have an image sensor.
There are many different types of sensors. Some are used in everyday objects, and some are separate.
Thermal sensors.
A sensor that detects temperature. Thermal sensors are in many laptops and computers in order to sound an alarm when a certain temperature has been exceeded.
Electromagnetic sensors.
An electronic device used to measure a physical quantity such as pressure or loudness and convert it into an electronic signal of some kind (e.g. a voltage).
An electromagnetic sensor is anything that can detect things like photons, electrons, magnetic fields etc. It can be as simple as a white wall, a sheet of white paper, etc., or as new as a charge-coupled device. When a laser is aimed at a detection screen, a bright spot can be seen on the screen, or when an electron hits the phosphors on another kind of screen, it makes that screen glow.
Motion sensors.
A motion sensor detects physical movement in a given area.