Here is a resources for Gels and Diffusion.  There are a variety of was to use these two alone and in combination.  Throughout each section there will be and interactive tutorial allowing you to experiment.  Each section you will learn how to use these as part of your craft and obtaining the particular look you are trying to achieve.

ARTICLE ON USING GELS

ARTICLE ON DIFFUSION

Below are images representing different gels and how they react in different environments on set and with a specific light source.

Below is a chart of different diffusion filters.  There are many other types of diffusion though we will only cover these types in this course.  The chart represents the amount of light visible when applied.