--TAKEN FROM A POST AT TERMINATORFILES.COM--

Early battlefield endoskeletons, such as the T-200-500 were probably not any type of "hyper-alloy" composite. They were probably made of regular iron and steel which were probably found all over the destroyed cities. Especially the very early ones. Regular fire arms were enough to take those suckers down after a few well-placed shots.

The T-600 was the first copy of the human skeleton. It was probably designed with reenforced steel with extra protection over the vital areas such as the power supply and N.N.P. (neural net processor, which is a first for Terminators in the 600 series).
The 600s are designed to be fast, agile, and as similar as true human motion as can be at that time. The "Infiltration Units" of this series was the first to appear as human as possible using latex covering and "tacked on" human hair. Up close the resistance stated that they could tell right away that this was a machine. But from a bit of a distance, especially well clothed and armored with Techcom "uniforms", it was much harder to distinguish, giving this infiltrator time to get close enough for some easy kills.
Plasma weapons, by this time, have become the battle standard for both sides. This allowed the resistance to easily take down machines with just a few contacts of super-heated plasma! The 600 series now had to use its "learning" capabilities in order to succeed in getting close to its targets. The earlier models just used standard move forward and attack. But now, Skynet had to develop new tactics for the war.

Skynet put out the "rubber-skinned" infiltrators as a beta test measuring its successes and buying itself some time in order to create its ultimate infiltrator units.
The 700 series was an upgraded 600 version. This endo was better armored being made from titanium so it was stronger and had longer lasting power supplies. The endo had better fluid human movements also. It was written somewhere that the 700 infiltrators used a simple cloned skin covering to appear human. Although it was just flesh. No muscles, veins, arteries, or body hair. So basically, it appeared to look just like the rubber/latex 600 series, but a bit of an improvement when viewed up close. Skynet was still buying more time as he was perfecting something better.

Then came the 800 series which we all know and love! This endoskeleton was designed to be an exact replica of a human skeleton, except made from a titanium hyper-alloy combat chasis. They are very durable, agile, super human strength and more plasma resistant. This terminator battlefield endoskeleton became the standard for the machines in the war due to its high intelligence, speed, and ability to manuever perfectly over a junk filled wasteland. These guys not only use infra-red to see, but are also equipped with night vision so they can "see everything".

The infiltrator version is the first to be fully equipped with outer living tissue, which included everything a human would (flesh, muscles, veins, arteries, etc.). These guys were (at first) the most deadly towards the resistance. They could totally pass for human being able to talk perfectly, sweat, and even have bad breath. Until the resistance learned to use dogs as "terminator detectors", these guys were able to sneak into Techcom bunkers as wounded soldiers, lost civilians, or just come up on the flanks of battle-involved resistance soldiers and start blasting away causing mass confusion.

Once Skynet released this series and saw the damage being done, it started to mass produce many different models. Each model had a different physical appearance and even a slightly different "personality" due to its very intelligent neural-net processor (a learning computer). Sometimes this would cause random "glitches" in the different types created. Meaning that sometimes, although not too often, a terminator would decide to just do what it wanted disobeying direct Skynet orders. The would become rogue and begin to fight both man and other machine trying to receive its own independance. It just wanted to learn about the world it was in. It would acheive a lowly form of sentinence in a manner of speaking.

This infuriated Skynet, and to solve the problem he created a switch on all the 800 series chips that would prevent certain aspects of the learning to peak. By adding these read only/write only/r&w modes, Skynet could prevent these machines from becoming rogue or achieving some form of independent thinking. It kept all terminators on the battle field in read only (although it could still learn certain aspects such as new strategic tactics), Skynet could have its machines do only what it wanted them to.
Plus, Skynet could switch these from fully autonomous to a form of remote access, thus allowing Skynet the ability to have direct control over these machines (similar to his ability to doing this with the HK units).

In the midst of developing newer and deadlier models, Skynet grew tired of the resistance scavenging the plasma rifles and cannons from incapacitated T-800s, so he tweaked their design a bit. Thus, the T-850 was made. This was just an upgraded 800 with only minor adjustments such as superior strength, able to withstand more plasma shots because it was made of a new coltan hyper-alloy, greater intelligence (psychology was added as a basic sub-routine), and a new power supply.
This new power supply would catch the humans grabbing destroyed machines weapons off-guard. Once the 850 series endos went down, any humans within the proximity area would be wishing they weren't. The T-850's power cells would become unstable and cause a very deadly blast, basically becoming a machine type of suicide bomber. This became another destructive blow to the resistance!

After Skynet learned about the resistance capturing terminators and reprogramming them to fight for them, it designed something to hunt and destroy these "traitors". So Skynet created the T-900. These machines were armored to the teeth including having it created from coltan, which has a much higher melting point. All of its machine "guts" were plated with this new coltan hyper-alloy combat chasis. Like the T-850, the 900 series is also powered by the miniture plasma fusion reactors that also become unstable when incapacitated. In order to infiltrate bunkers, Skynet equipped this model with an ultra-sonic high-pitched tone that would drive the dogs (terminator detectors) crazy to the point that sometimes the dogs would start to attack everything even their owners.
Even with its new capabilties, the 900 series was still not very effective in battling other reprogrammed terminators since it was too similar. Skynet had to think of something else.

Skynet wanted to try something brand new after it learned about nano-technology and its possible perks in infiltration. This lead to the creation of a limited prototype design called the T-1000. This new creation was a Mimetic-Poly Alloy machine. It was made from billions of microscopic nanomachines able to appear in a liquid or solid metal appearance. It was virtually indestructable unless one could hit it with multiple high temperature plasma cannon rounds. It could appear as any same sized shape it wanted to as long as the memory sensors came into direct conact with it just like a chameleon.
The T-1000 could have its nanos go from a flowing liquid to a sharp and hard solid in seconds. This allowed it to make stabbing weapons out of its limbs at the drop of a hat. Infiltration into groups was very easy since it could make itself appear as a well known friend after terminating that subject. There weren't that many created (because of it being very resource heavy), but from the amount that was, it was a deadly weapon which racked up high body counts.

The down side for Skynet was that it could only be autonomous. This slightly scared him because if these creations decided to turn on their creator, then Skynet didn't even know if it could stop them.

With the 900 series unable to put a dent into the growing number of reprogrammed terminators, and it being scared of a possible T-1000 revolt, Skynet needed the ultimate weapon it could control against other machines. Thus, the T-X was made. Using a heavily modified 900 series design, it created a terminator that the resistance couldn't scalp if incapacitated, and one that could have all the advantages of the other models, but none of their flaws.

The T-X was a battle-hardened hyper-alloy combat chasis that was covered in nanotechnology making it poly-mimetic. Each arm of the endoskeleton was able to turn into many different designs of onboard weaponry. It also had nano-disruptors that were able to be injected into other machines. The T-X could then control the target via remote link-up. This design was also able to see with a greater ability than the others.
The T-X was the ultimate killing machine for destroying both humans and other machines. The T-X is virtually indestructable, but Skynet could be able to control this model if it had to due to the fact of it having a physical endoskeleton design he/it could tap into.