What is the relationship between Caesar and Brutus

What is the relationship between Caesar and Brutus. However I don't mean how are they connected in the play. What I mean is literally, "what is their relationship?" Were they best friends, were they connected through marriage, were they just apart of the same government, ext.? The main question I'm trying to get to the bottom of is, why was caesar so shocked when he saw that Brutus was apart of the group killing him? What the emotion and meaning behind his famous words "et tu brute!"(and you brutus!)