A comparator chip has two voltage inputs, and outputs a voltage based on which of the two inputs is higher. There are many ways to configure these kinds of circuits and there are many uses for a comparator as I'm sure you can imagine.
This video goes through the main configurations for comparator circuits and discusses some of the different ways that you can set a reference voltage for the comparator.

For more operational amplifier topics, see the website
www.electronx.ca/education/op-amps/
here you can find videos, tutorials, example problems and more on op amps, electrical and electronic circuits.