Angry chickens will drill you with their beaks and scratch you with their talons. A rooster may attempt to stab at you with his spurs. You can get anything from a pinch that doesn't leave a mark to a bleeding scratch, depending on the type of attack and how protected you were.
These are natural dominance behaviors. Aggressiveness can be a good thing, especially in a rooster that supervises his flock while they free range. He is likely to challenge something he perceives as a threat.
It is your job to assume the role of "top rooster" in your flock. When your rooster shows signs of aggression, don't run away like a lesser bird would. Lunge for him and grab him (you may need to wear long sleeves and gloves for this). Hold him upside down until he stops freaking out. Carry him around. Pet him. Talk to him. Feed him treats out of your pocket. Never let him get away with attempting to attack you without doing something in retaliation (you don't need to hurt him or "punish" him, but grab him so that you assert your control over him).
Hens are less likely to attack you unless they are sitting on a nest. You are likely to get drilled with a sharp beak if you try to reach under a protective hen. Wait to gather the eggs until the hen is done sitting on her nest, or shoo her gently off the nest (scooting her gently with a hand whisk broom works well) if she is trying to go broody.
Certain chicken breeds are more likely to show aggressive behavior than others:
http://www.ithaca.edu/staff/jhenderson/chooks/chooks.html
Of course, there are always exceptions.
