Via
Beginner
A tiny vertical plug that connects a wire on one layer to a wire on the layer above or below, like an elevator between floors.
Novice
A small vertical metal plug through the insulator that joins a wire on one layer to the layer directly above or below. It has a cut (the plug itself) and a metal landing pad on each side. Each via adds a little electrical resistance and takes room that nearby wires could have used.
Expert
Defined in LEF and placed by name in DEF. Rules cover the spacing between cuts, how far the landing pad must extend past the cut (often different in each direction) and a minimum number of cuts on wide wires. Via resistance adds delay, which motivates double vias, via pillars and routers that count vias in their cost.