This port type allows you to write custom Cg code. The shaders runs on the graphics card and allows you to directly change the vertices and rendered pixels. Standard library functions that exists in Cg can be found on Nvidias homepage and a older index but with a quick overview can be found at Cg Tutorial - Standard Library Functions

For a very good explanation of what shaders are and how they work see upvectors tutorial
they also have a equality impressive procedural textures tutorial

External links to further reading about how a few specific effects are done

• Xor demoeffect
• Tunnel effect
• Raycasting two planes
• Twist effect
• Ripple effect
• Sphere effect