Ultra Droids are automated infantry drones that serve as the backbone of Doom's military forces. They have no built in weapons, instead using the same weapons as Doom soldiers. They are capable of squad-level infantry tactics and understanding vocal commands.
These androids somewhat resemble Drules, appearing to wear charcoal-black armor. In fact, some of the living soldiers appear identical to the ultra droids. 
They serve as an effective force multiplier for infantry, as a squad of operators can have the combat capabilities of an infantry company. In addition, ultra droids can be used for scouting and minesweeping and entering environments full of nuclear, biological, or chemical hazards, are more durable, and have no fear of death. On the other hand, they are even more reliant on communications networks than their living counterparts, and have less capability to adapt to changing battle conditions.
Presumably, they have firewalls to prevent hackers from changing their mission parameters (like having them kill their operators). On at least one occasion, the firewalls were breached.