Right here goes :
GBH is worst. GBH = Grievous Bodily Harm and ABH Actual Bodily Harm.
The charge will usually wait until a determination of the healthof the victim is made. i.e. the attacker will be bailed/remanded until it is definite. If they die Murder/Manslaughter charges will be addressed.
Critical means that the persons life may still be at risk but the doctors have stabilised their condition using whatever means.
The sentances are wide and varied depending on the persons previous convictions and severity of the offence etc - there is no cut and dried sentance for either - could be a fine or maximum of See here :
http://en.wikipedia.org/wiki/Grievous_bodily_h arm
or here for abh :
http://en.wikipedia.org/wiki/Actual_Bodily_Har m