Section 1.8 Switches (WIP)
A switch controls the flow of current by mechanically connecting or separating metal contacts.
A simple switch has two states: open (where there is a gap between the contacts, preventing the flow of current) and closed (where the contacts are connected, allowing current to flow).
You can think of an "open switch" like an "open circuit": there is a gap in the circuit that prevents current from flowing.
If it helps you remember the terminology, here's another analogy from Kenn Ahmdal: imagine that electrons flowing through a circuit are like ants walking along the top of a fence around a field. If the gate is closed, their path is complete and they can walk around and around. If the gate is open, the ants must come to a stop.
Subsection 1.8.1 Switch variations
There are lots of different variations of switches. Don't try to memorize or absorb all this at once; just make note of it and come back to this as a reference.
A toggle switch
A momentary switch has a mechanical spring that causes it to return to a specific position when not pushed. A mommentary switch is considered normally open (often abbreviated in schematics or on datasheets as "NO") if the switch is open in the resting position, and pushing the switch causes the contacts to close. Conversely, a normally-closed ("NC") switch is closed in its resting position, and it breaks the circuit when pushed.
It is fairly common to sell switches with terminals for both NO and NC, making it possible to wire them up either way.