Edge Split is one of Blender’s modifiers:
It will happen quite often that you have smooth (rounded) and flat areas on the same mesh and also sharp and smooth edges. But whenever you hit the smooth and flat shading buttons in the left-hand “T” panel, Blender will apply those shading modes on the entire mesh, which will result in flat areas looking weird because Blender tries to smooth the unsmoothable or - on the other hand - rounded areas appearing facetted with shading set to flat.
The Edge Split modifier splits edges within a mesh and - as a side effect - makes edges appear sharp when their angle is above a certain threshold. This more or less automatically takes care of the smooth/flat/rounded/sharp issue:
This is basically a cylindrical object with some cubical extensions. Smooth shading alone (right) looks plain weird and will also spoil the shading when rendering. With Edge Split (left) all is shiny.
Concerning node groups:
Select the nodes you want to re-use and hit Ctrl-G. You can now rename that node group to your liking and whenever you hit Shift-A to add new nodes, you will see a new “Group” category from which you can insert your custom node groups.
Or you just Ctrl-C (copy) and Ctrl-V (paste) the nodes to where you like.