In James Camerons 1984 classic The Terminator Kyle Reese told Sarah Connor of the T-600's stating "The 600's had rubber skin. We spotted them easy.". He then related to her about the T-800's "But these are new; they look human - sweat, bad breath, everything. Very hard to spot". Then, in the 1991 sequel we were introduced to the new Mimetic Polly-Alloy prototype the T-1000, that could "...imitate anything it samples by physical contact".
So, for Terminator Salvation director McG wanted to show Skynet's process of technological evolution by showing us the series 700 Terminator, or T-700; which was to be a bridging model between the 600's and the 800's. But the movie was plagued with production problems and script re-rwites, leading to some confusion in the final, finished theatrical version of the movie.
Contrary to John Connor's observations the Terminators being constructed in the factory at Skynet Central, San Francisco in 2018 (the setting for Terminator Salvations climax) are not T-800's but are in actual fact the T-700's.
Though the 700's do look very similar to the 800's there are a few differences that separate the two. Firstly the 700's are constructed from existing materials like steel and titanium, much like the 600's, which causes them to blacken and rust easily. Whereas the 800's are constructed from a "Hyper-Alloy combat chassis; micro-processor controlled.", which is not only structurally more superior but lends the 800's a chromed, silvery shine. And finally, in addition to some slight structal differences between the two models, the 700's teeth are metal just like the 600's, whereas the 800's are fitted with human teeth, taken from captured human refugees. this can clearly be seen in the comparison pics below...
Left to right - T-600, T-800, T-700
The T-600's where used by Skynet after the fallout from judgment day as combat units. In an attempt to trick human forces the 600's were dressed in human clothing and a plastiic mask, but because of their size, bulk and inability to speak Human Resistance soldiers were easily able to spot them. Skynet then started mass producing the 700's in 2018, while developing a prototype T-800 AKA the T-RIP (Terminator - Resistance Infiltrator Prototype). The destruction of Skynet Central ceased production of the T-700's, with Skynet instead concentrating on amassing an army of the more advanced, more durable and more intelligent T-800's; both as foot soldiers (as seen in the Future War sequences in Terminator 2 and Terminator 3) and as infiltrators (as seen in the second Future War dream in The Terminator).
