Very Basic Descriptiion
The Isoceles basically is drawing the gun, putting both hands on it and pushing your hands away from your body until they stop. The two arms and your chest form an isoceles triangle.
Now, if you were holding your gun out in an isoceles, turn your weak side should in toward the center of your body (just the shoulder) so that your weak side arm is bent and your strong side arm is straight or close to straight. Your weak side hand should be pushing back toward your body, and your strong side hand should be pushing out away from your body. So your hands are pushing together with the gun in the middle locking it in.
In both cases, your head should be held upright in a normal position like you were just walking down the street.
That is the very basics. In both stances the feet are parallel to the target or bladed slightly toward the strong side.
The isoceles is very popular with action pistol shooters. The Weaver is advocated by a lot of combat shooters. The Weaver was championed by Jeff Cooper and is part of what he calls the Modern Technique of the pistol. Here is a link with some basics on that subject:
http://home.sprynet.com/~frfrog/modtech.htm
The story of the Weaver goes like this. Back in the 50s Jeff Cooper and some of his buddies started having pistol matches. They wern't bullseye matches, but were very early attempts at action shooting games. They called these matches leather slaps, and they were held in Big Bear California. They were using holster rigs like you would see in the old western movies and we doing stuff like point shooting. They were having a lot of fun until a guy named Jack Weaver showed up and beat the pants off of all of them. He didn't shoot like they did, he used what is now known as the Weaver stance. Jeff knew a good thing when he saw it, and worked with it for years, I am sure refining it a little along the way.