Here is a simple explanation.
Imagine you go back to the Zhou Dynasty, and became the leader, you found it is hard to manage the big territory without cars and planes, so you enfeoff your territory to many relatives and friends, they manage it for you.
You are 天子, your relatives are 王, and your friends are 公, and then 伯 and other lower titles. Besides the title, they are all the leaders in their own country, yes they are countries, but the leaders obey to you since you are 天子.
Now you go to Qin Dynasty or any following dynasties, you don't want to enfeoff any more, cuz they all want to steal your 天子 title. Now you have to manage your whole land by yourself, then you think, I am the greatest ever, I am 皇帝.
But sometimes, you like some friends very much, you may give him the rank of nobility 王, but there are many differences between this honor 王 and the enfeoffment 王.