Ranking / Personalization

How Hacker News ranking algorithm works

How Reddit ranking algorithms work

Let me guess What You Like the most - Recommender Systems...

Categories for YouTube Video
Types and Categories of Videos On YouTube

The above mentioned categories are just a few of the many that you can found on YouTube. Additional categories include 

- arts and animation, 

- autos and vehicles, 

- news and blogs, 

- people, sports, and

-  science and technology.