In my experience, there appear to be 3 levels of parrying, which I call "near parries", "bad parries", and "good parries".
Near parries occur when hit when the parry frames have ended, but the parrying animation is still occurring. Near parries are treated as a block instead of a parry, but only block half as much as a normal block would (if a shield has 70% physical block, a near parried attack will be 35% blocked), and drain more stamina than a normal block. They also will not put an enemy into a riposte state. NOTE: Fist and claw weapons, having no defensive attributes, will enter instability frames instead of near parrying if the attack hits after the parry frames have ended, causing the user to take more damage than if they had simply been hit by the attack. They will also immediately reduce stamina to -60 upon a near parry.
Bad parries are a halfway point between near parries and good parries, taking place on what appears to be the second half of the parry frames (I have not tested this extensively, it may be more or less than half). Similar to near parries, they will block half as much damage as a normal block would, and drain more stamina than a normal block would. However, they are still considered to be parries, and will still put the enemy into a riposte state. Fist and claw weapons will not enter instability frames if a bad parry occurs, but the user will take full damage from the attack and the user's stamina will be reduced to -60, making a riposte difficult to pull off.
Good parries, are, well, what we want when we parry. They happen during the first half of the parry frames (again, the exact ratio is not known to me, I've just been eyeballing), and the user will take no damage and lose no stamina when hit (they will, however, still lose the stamina for performing the parrying animation, of course). There is no significant difference between a good parry on most parrying tools, and a good parry on fist and claw weapons.