Okay, here it goes.
T-1
T-70
T-600
T-800/850
T-900
T-1000
T-X
T-1000000
T-1: Machines in T3 that look like miniture HK tanks. They are armed with twin miniguns. Officials sources incorrectly cite these weapons as .50 caliber machine guns that fire depleted uranium rounds. In reality, they are .30 caliber weapons.
T-70: These machines are the first to actually look like Terminators. They are bulkier and less human-like so they have no hope of any sort of skin covering. On the right arm they have a Gatling gun weapon (ficticious) and were designed to be an artificial soldier. These are from T2:3D and probably no longer fit in the official Terminator storyline.
T-600: The T-600s are Terminators that have rubber skin. It is unknown whether the endoskeleton is the same as the T-800.
T-800: The T-800 is the first and only true cyborg. It is the Terminator that Arnold plays that has real flesh over a metal endoskeleton. In T3, Arnold plays the slightly upgraded 850 model that is a bit hardier (resistant even to plasma attacks).
T-900: The T-900 is a chassis in between a T-800 and T-X. Basically it's a more solid design that allows for more armor. Interestingly enough Arnold himself plays a T-900 instead of the standard T-800/850 in some of the Terminator 3 video games, for reasons unknown.
T-1000: This is the advanced prototype that appears in T2 and T2:3D. It is a liquid metal machine (more likely a nanoswarm that has variable viscosity) which allows it to mimick smooth organic shapes (such as people or stabbing weapons). For whatever reason the T-1000 has a "base shape" that it usually reverts back to. This is more than likely for the audience's convenience.
T-X: The T-X is something of a fusion between a T-800 and T-1000. It is basically a new, compact, extremely advanced chassis with an outer covering of liquid metal. This gives it a limited shape-shifting capability (though since the machine has a fixed frame, it is limited in it's ability). The T-X also boasts complete flexibility able to rotate most joints 360 degrees with full freedom. Finally the T-X has built in weapons, including nanotechnology that can overpower and commandeer enemy computer systems.
T-1000,000: The T-1000,000 or T-Meg is a giant liquid-metal spider that guards the SkyNet core. It doesn't take the shape of any particular human so it mainly just uses the regenerative abilities to it's advantage and eliminates opponents by using it's tremendous bulk to drive heavy spikes into them. This guy is in T2:3D and thus may not be canon to the story, but unlike the T-70 it isn't a direct contradiction to T3, aside from there "being no system core" which seems to be refuted in T1 when Reese claims "their defense grid was smashed."
There are surely other Terminators. Some websites detail T100s, T200s, T300s, all the way to the 700s. These are either fan speculation or appeared in novels and comics and other (presumably) non-canon material.
====
Here is the official explanation.
"101" is the Arnold skin type. So if it looks like Arnold then it is a Model 101. T-800 or 850 is the skeleton.
So in Terminator 1 and 2 Arnold is T-800, model 101. In Terminator 3 he is a T-850, model 101. In Terminator 3: The Redemption (a video game) he is a T-900, model 101.
Add T-400s which were roughly humanoid robots sent into demolished bunkers to clear away any survivors. They were unintelligent, slow moving and prone to rust. Early in the war it was a common sight to see a platoon of them being led across a battlefield by a more intelligent T-500. Since then they were recommissioned for lesser duties, guarding SkyNET prison camps.
Sometimes I like to pretend the second and third movies don't even exist. The first film makes it clear that the T800s (Arnold's model) are the newest, most advanced and baddest machines. Then Terminator 2 comes out. Okay, I can buy one advanced prototype. Then Terminator 3 comes out and we have the T-X who could school both of them, if it wasn't for movie contrivances. Terminator, like all franchises suffers from inconsistencies.
I'm sorry for bumping this thread, but I rarely go outside the Star Wars forums, but when I read the response here, i was amazed that someone actually gave such a detailed answer. Also i had to comment on the inconsistencies of the series...by the movie's own logic, T2 isn't even possible. Reese couldn't bring back any "ray guns" because nothing dead could come through, and when asked about the machine, that was possible cause it was covered with "living tissue". There is nothing living about liquid metal, and no way that a prototype terminator could change his molecular structure to even APPEAR like living tissue. I'm not a T2 basher, cause i love the movie, but by their own rules, it couldn't have happened.
And if there's any little fan boys that read this, please don't try to bash Star Wars to piss me off, I know its chalk FULL of plot holes and errors. And I'm bashing terminator like i said, it's one of my favorite movies.