Saber is used to express concrete facts or skills. For example, "Él sabe mucho sobre matemática."
Conocer means 'to be familiar with' or 'to know' in a more contextual way. It applies mostly to people, places, or things. For example "Conoczo una enfermera."
Now, let's see how they differ in a sentence.
"Conozco una bibliotecária que sabe mucho sobre los libros."
I know a librarian who knows a lot about books.