What is the difference between ?

Asking what the paragraph is about usually mean to explain what you just read so the teacher knows you understand it. Offer a quick summary.

Example: What was this paragraph about? This paragraph was about the way different people perceive ice cream flavours.

Asking what the author is trying to say can be used to mean the same thing. More often though, it's referring to a very small and specific section of text. It also often calls for an interpretation of the subtext. Example: What was the author trying to say by using the word rich to describe Chocolate ice cream? The author was trying to say that Chocolate ice cream is often very sweet and has a strong flavour. 

The difference is as thin as one single hotcake.