The archetype of the Shadow is pretty much the 'bad guy.' This is the person/thing that the hero must end up fighting, the hero is compelled to fight he/she/it. This is a very common archetype found in everything that has a hero. This archetype doesn't follow the rules and just hates authourity and ends up being their own boss.
The Shadow is also described as the dark side of the hero. This is the side that the hero denies in himself that they project onto others, the side that split from the hero. the Hero is then compelled to fight the Shadow in order to reintegrate with that missing part and become 'whole' again.