To "know of" simply means to know somebody or something exists, but implies that you don't really know the person or the thing. There are several examples here, and I'll try to explain each. "I know a different route to get to work" means you are familiar with a different way to get to work. "I know of a different route to get to work" means you simply know that another way to work exists, though you likely aren't too familiar with it. "I know Charlie, your sister's boyfriend" means that you know and probably have met Charlie. "I know of Charlie, your sister's boyfriend" definitely implies that you have never met Charlie, but you do in fact know that the sister has a boyfriend named Charlie.

'Know of' is used when you have personal experience with what you are talking about