There are many different kinds of sensors that can be used for many different things. However, most sensors can be put into two main groups: passive and active.

Passive sensors are those that detect and measure already-present forms of energy in the environment and don't need any energy from inside. For example, our eyes and ears are passive sensors because they pick up light and sound that comes in (both forms of energy).

Active sensors are sensors that send and receive their own signals using energy they make themselves. They need this energy to work. Dolphins and bats, for example, use echolocation to find their way around. They do this by sending out pulses of energy that bounce off objects and are then picked up and used to figure out what's going on around them.