A war orc can only be generated in Gehennom or in Grund's stronghold. It is the weakest of the new orcs, and has no particularly striking features. It is unlikely to be a threat, even if encountered in Grund's stronghold.
A great orc can only be generated in Gehennom or in Grund's stronghold. While they have a fairly large amount of hit dice, they lack any special abilities and have only a modest melee attack.
Snow orcs are occasionally found in the middle dungeon or Grund's stronghold. They can be be a modest threat due to their appearing in large groups and using weapons, although a player with speed can avoid them. They are cold resistant, and their corpses have a good chance of conveying that intrinsic. They do not have any other special abilities, however: their attacks are not cold-branded.
Demon orcs can only be generated in Gehennom or in Grund's stronghold. They have a high chance of granting fire resistance, and resist fire. By the time one sees them, they are unlikely to be a threat, although early travelers to Grund's stronghold might wish to be cautious.