What is the difference between a third person limited narrator and a third person omniscient narrator?

Omniscient means "all knowing". If you are reading a book and the narrator is omniscient, he knows how everyone is feeling and what everyone in the book is thinking.

For example:

John was feeling hurt because of what Sarah had done. He said, "I thought we were friends," This did not matter to Sarah.

Third person means that there is just another person telling the story, if we are talking about a book. You only know what they know.

For example:

John looked hurt, then said "I thought we were friends," It did not seem that Sarah cared.