It is useful to distinguish static operation nodes from all other operation nodes. They have a lighter look that indicates that they are not operating on a state, ie. they do not store any data between consecutive calls.
A process node represents a single instance of a patch. The name Process comes from the fact that it can be understood like a little machine that continues to run (update) after it was created and saves its state from one frame to the next.
See also Datatype Patch.
Nodes with exactly two inputs named Input and Input 2 and one output named Output can automatically have additional inputs added/removed by pressing CTRL + or CTRL - respectively.