Links are the connections between pins. There are 3 different kinds of links.
Links can have any number of link points that can be added while a link is made. Link points can be selected, moved and deleted individually, ie. independent of the selection of the link.
Links can be deleted with a middle-click.
Finishing a link on an input with a middle-click starts a new link from the same source.
By default links are not included in a marquee-selection when other elements (nodes, pads, regions) are already in the selection. To force the selection of links in such scenarios press SHIFT.
By doubleclicking an existing link you bring up the NodeBrowser to choose a node that will be inserted in the link.
While making a link you can:
doubleclick to bring up the NodeBrowser and choose a node to be connected to the link
Ctrl click to create in input or output pin
Shift click to create a pad
Alt click or middleclick to create an IOBox.