MATERIAL DESIGN
Material design was developed by google. It was first introduced in year 2014 with the launch of android lollipop. Material design can be seen in mostly applications designed for android and also in chrome os. Many websites uses material design system like flipkart,facebook,pinterest and may more.
If you are an android user then you have seen it in many applications.To know more about material design see there official website
FLUENT
Fluent design was developed by microsoft. it was launched with windows 8 and continued by windows 8.1 and windows 10.
To know more about fluent design in detail visite there official
APPLE
Apple has created a simple and unique design system. without the use any drop shadow effect like material design do it has created an minimalistic design system.apple is the best example of unique style.
to know more about apple's design system take a look at their website