"The automated units were not created naturally. We were built."
Programmed to fight in a war being waged between the two groups, both types of units were of a very similar design: humanoid in form with "faces" having only the most basic of features. The primary external difference was that the Pralor units had a silver colored metallic covering while Cravic units had gold colored ones. Pralor units included designations 3947 and 6263 while Cravic units included a designation 122. Both Pralor and Cravic units were also designed with chromodynamic power modules – power sources intended to be non-reproducible as a safeguard against the androids becoming self-reproductive. Automated Unit 3947 displayed the ability to discharge chromodynamic energy through his hand as an offensive weapon powerful enough to at least knock two people unconscious.
Eventually the Pralor-Cravic war came to an end and both sides agreed to deactivate their Automated Personnel Units. The units on both sides, sensing a threat to their existence, turned on both the Pralor and Cravic, exterminating both races, and resumed the war.
In 2372, the USS Voyager discovered a non-functioning Pralor unit. B'Elanna Torres reactivated the unit, and was able to determine a means to reproduce the chromodynamic power module such that other androids could be powered. When she realized that allowing the Pralor units to procreate would vastly alter the balance of power in the conflict (and possibly the quadrant), she destroyed her work and allowed the remaining automated personnel units to continue their war. (VOY: "Prototype")
Known Automated Personnel Units
The Automated Personnel Units were created out of separate vacu-formed components that were fitted over the actors to allow them full movement and flexibility. (Star Trek: Aliens & Artifacts, p.169)
Even though none of the units are specifically assigned a sex, their voices sound distinctly male, as they were voiced by the same two actors – namely, Rick Worthy (who played both 3947 and 122) and Hugh Hodgin (who played both 6263 and Prototype Unit 0001).
Ken Biller said of the challenge involved in creating the robots, "That was an almost impossible task given what our budgetary constraints are and given the time that is alloted. We couldn't afford to do body makeup so suddenly he's in costume." (Cinefantastique, Vol. 28, No. 4/5, p. 92)
In a 2010 interview, Rick Worthy recalled working in the costume; "[It] was a lot of fun, but really hard too because I couldn't see anything inside the robot suit. They put two little slits where I could barely see out. I had to practice where my marks were on the floor and when the cameras rolled I had to force myself not to look down. When it was over I had to voice over the whole thing, because I was speaking from inside of a mask and the sound wasn't recorded very well." 
The small eye-holes in the robot costumes dictated the kinds of shots that "Prototype" director Jonathan Frakes could use. As Rick Worthy explained, "Jonathan had to set up each shot in such a way that made the slits unnoticeable to viewers." (TV Zone, issue 126, p. 28)
Similarly, Ken Biller was unhappy with how the costume looked; "The robot looked like the guy in the suit. It just looked really, really hokey and cheesy [....] If he is a mechanical man, why is he wearing a suit? He looked like the tin woodsman and it was unfortunate and disappointing." (Cinefantastique, Vol. 28, No. 4/5, p. 92)
The Star Trek: Myriad Universes novel Places of Exile makes reference to the chromodynamic power module the Automated Personnel Units possessed. In an alternate timeline, Torres' experience with them makes her perfect for a mission to a Casciron ship where she would hide aboard after disabling the Vostigye vessel, Ryemaran.