This is not an uber-great article, but it's alright. He makes some good analogies that explain pixel and vertex shaders much better than most explanations I've seen before.
Look more closely at the water in the Half-Life 2 screenshot above. (192k, 1280x934 jpg) Notice the realistic dimming and distortion of color and pattern through the water. That's pixel and vertex shaders in action. Effects like that were previously almost impossible.