Material Output Node
At any point, you may want to see the work in progress, especially right after some operation by a node. Simply create another thread from the output socket of the node to the picture input socket of an Output node to see a mini-picture.
Connect the alpha channel to set/see transparency.
Effective Output Node
The only Output node that is used for the Material in the end (i.e the only non-Preview) has a little red sphere on the upper right.