Noob Ques: Composite vs Viewer?

What is the main difference between Composite node and Viewer node?

I know the Viewer lets you see any node with SHIFT CTRL click, and Composite is necessary for final result. But is there anything else more advanced that I am missing?

Thanks!

There’s one or more “pipes” that “pour” information into the noodle, and just one “drain” that takes the finished information out. The Composite node is the collector of what ultimately will be “produced.” (What finally gets drained into it, is your output.) Meanwhile, you can use Viewer nodes to easily examine what’s going on at any intermediate point in the process.

The viewer node result is shown in the backdrop or UV/Image editor.
While you should only have one Composite node in the tree, you can create multiple Viewer nodes and click through them to view quickly your compositing steps.
I like the SplitViewer node, which lets you view alternate composite render results side-by-side.

Don’t forget the File Output node, which saves automatically whenever you render and can be put anywhere in the tree to save intermediate steps.