The major difference in GLSL is in that it allows you to link shaders into a final program object binary. All previous technology required that you send the entire source code for the vertex program, compile it, and entire source code for the fragment program, compile it.. and with each render you have to enable the vertex program and enable the fragment program. With GLSL you can upload the source code for as many vertex shaders and as many fragment shaders as you need, compile them, then link them all together. Much more like a traditional compiler. This is good, because we can make pipeline programming modular now. Well, we can do so much more easily now anyway. To do so in DirectX HLSL you need to do manually "compile" the source code together from multiple string sources before you upload it and really compile it. That's a lot of trouble.