Would you explain the difference between the following?
I know of somebody/something.
I know about somebody/something.
Yep. This is the best example:
I know of atomic physics (I have heard the name, but that is about all I know.)
When you say know of someone/something, it means to know that someone or something exists but don't know very well.
I know about atomic physics (I have learned this subject and can do the calculations.)
When you say know about someone/something, it means to know that someone or something exists and you know very well that thing/person.
My interpretation of a few variations:
I know of The Spanish Inquisition.
I know of Richard.
I've heard of it/him. I am aware that it/he exists. But really the only thing I know about it/him is its/his name. When speaking aloud, typically the "of" gets emphasized.
I know about Astronomy.
I would not typically say this, as I think it doesn't carry any meaning. If I did say it, the next question would just be "How much do you know about it?" Better to come right out and say "I know a lot about Astronomy" or "I know a little about Astronomy" or some variation.
I know about Susan.
Implies that there is some specific knowledge about Susan that is known to you. If you have a daughter named Susan, and were hiding that fact, someone could say to you "I know about Susan".
I know Frank.
This means at least: I would recognize him if I saw him; I've interacted with him before; he is an acquaintance. But it does not exclude the possibility that you are relatives or good friends or lovers or anything else.