I think the 'cause of death' would mean the actual medical/biological/pathological cause of the body ceasing to function. This would be revealed by the autopsy. The 'manner of death' would be whatever it was that led to this.
A manner of death might be being hit by a car, and the cause of death might be a massive internal haemorrhage.
I'm not a doctor or pathologist, so this is just a guess.