We as humans have our own natural senses: sight, hearing, smell, taste, touch and many more, like balance and acceleration.
Computers are not as fortunate as we are and need sensors to make sense of the world around them by measuring and keeping track of different values in order to be able to react accordingly. They measure signals from the energy domain and translate them into computer-readable electrical signals.
The complexity of these sensors can vary in complexity and adaptability from a simple push button, which can be used everywhere, to a highly elaborated laser tracking system. When prototyping it can be easy to loose track and overview of what sensors exist and can be used to give one's project the desired interactivity.
The list of sensors is nearly endless, and if you google for them you will be able to find a fitting sensor for nearly every specific need.
Here is a selective list of available sensors commonly used in interactive projects:
What is a Sensor? Different Types of Sensors, Applications
Overview of different parts and sensors