A touch sensor is an electronic device that detects touch or near proximity.  They are also known as tactile sensors. Touch sensors are small, simple, and low-cost.  They are used to: 
Interact with electronic systems in an intuitive, tactile way
Control devices
Take input from the user
Provide an intuitive user interface
Touch sensors are used in: 
Lamps
Touch screens of mobile devices
Wearable and IoT products
Smart homes and appliances
Security and industrial solutions
Consumer tech devices such as smartphones and tablet computers
There are two common types of touch sensors: capacitive touch sensors and resistive touch sensors.