The Heretic is the builder unit for Chaos Space Marines. These Chaos Worshippers are damned, forced to build the blasphemous structures for their unholy masters.
Just like the Servitor of the Space Marines, the Heretic is the builder unit. He can be reinforced up to a squad size of 3 which lets them build structures faster. Heretics have no combat capability but they're relatively quick and can a cheap expendable scout unit very early. They cannot be selected alongside the main units except for other heretics.
Heretics can be forced to work harder, using the Forced Labour ability. This makes the heretic build and repair faster at the cost of HP. So if you activate the ability and forget to turn it of, the Heretic simply dies through overworking. And just like in the Space Marine Faction, the Heretic also has a "bigger brother" that is known as the Hellsmith.