Nodes tutorial or reference guide?

Perhaps there is a better forum area for this, but since Nodes are present in all areas I figured I’d start here. I feel lost when it comes to understanding when to use what Node for what. Tutorials can be followed to create specific effects, but was wondering if there is a series of tutorials just on nodes that starts out with what each one does individually and then builds up to combining the output of one Node with the input of another Node, building further by mixing them and so on?

A pretty good reference from when nodes were new : http://wiki.blender.org/index.php/Doc:2.4/Manual/Composite_Nodes/Types

They’re really somewhat abstract (even with the documentation) until you play with them and find out what they can and cannot do. Just try to keep in mind this: for most node based operations you’re simply manipulation grayscale values in one way or another.

The following page gives an easier to digest description that makes sense if you really study it’s geek-speak:

http://www.blender.org/development/release-logs/blender-243/render-passes/

Thank you for the links, they were very useful!