A war orc can only be generated in Gehennom or in Grund's stronghold, and also replace dwarf kings in the Gnomish Mines if the PC is a dwarf. 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.