Sensor
Electronic device or object capable of detecting real-life senses (smell, sight, hearing, taste, touch) and interpreting what's detected into data that the device can understand.
A good example of a sensor is a motion sensor that connects to a computer to help detect motion within a building. Other sensors may detect temperature, light, moisture, pressure, radiation, and sound. The picture shows a Verifi fingerprint sensor and reader, a hardware device that lets you identify yourself with the computer using your finger.
How is a sensor an input device?
Like all other input devices, a sensor sends the information it senses to the computer as input. For example, in our example of a fingerprint reader, the sensor sends the data it gathers from reading a finger to the computer. If that data meets certain requirements, the computer unlocks, if not, the computer remains locked.