Hi wscottaart,

To get a clearer understanding of what is going on, uncheck the "Filter Simple Shaders from tree". This will show exactly what is going on between the two channels that are being combined in the color branch of the shader tree. The difference is one of personal taste. All of the mixing options are found under the "Operators" option in the shader pull-down type. 

Just as in Photoshop, the Overlay, Add, Subtract, etc. give your different ways to combine what you have going on with the two layers/branches that are within the mixer option.

Mark