There are two categories of physical sensors, active and passive.
Active sensors - The sensor emits some form of energy and then measures the impact as a way of understanding the environment. These sensors often have restricted environments.
Some examples of active sensors include ultrasonic, laser, and IR sensors.
Passive sensors - The sensor receives energy already in the environment. These sensors consume less energy, but often have signal and noise problems.
An example of a passive sensor is a camera; it uses the energy in the environment to capture a snapshot.