Both slang. Both similar. Bro is a general term of endearment, like dude, mate etc. Bruh is another way of saying bro, used by African Americans, for example "Wassup, bruh!".
I guess bruh is sometimes kind of used as a substitute for "really" or "seriously", for example:
Guy 1: "I told her I loved her and she ran..."
Guy 2: "BRUH"