What is the difference between All and Everything?

All: The whole quantity or extent.

Is nearly always used with a noun, but there are some exceptions to this rule.

Example:

Did you eat all the candy? ✔

Did you eat all? ✘

Everything: All the things of a group.

Everything is a pronoun and means each and every one thing.

Example:

I explained to her five different topics, and she understood everything. 

It means she understood every single one of those five different topics.