Links
Links are the connections between pins on which data flows from one node to another. There are 3 different kinds of links:
Image:Normal Link, Reference Link, Delegate Link
In datatype patches, links can have colors, which tells you which member operation they belong to.
A "yellow sock" on a link is a warning that the source of the link is mutable and it connected to more than one downstream node. Please read the full explanation in the tooltip to learn what this means and how you can deal with it.
For many different actions you can do while creating a link, or on existing links, see Link Shortcuts.