In a few words it kinda impossible to define a switch.
Some of the more important things are:
1. Switches are rated for AC or DC or both. If you don't head the rating, the contacts can weld.
2. Some switches have an inductive or horsepower rating.
3. Some, rare, must not switch a current carrying circuit.
i.e. the switching current capacity can be much lower (including 0 Amps) than the on capacity.
4. Switches that switch low current have special plating such as gold
5. Switches can be designed to remove the oxide of the contacts mechanically.
6. Some switches can have arc suppression.
7. There may be a wetting current specified. A minimum recommended current to switch,
Now, there is a pole designation or how many contacts move.
SP (1P), DP (2P), 3P, 4P; single pole, double pole/2 pole, 3 pole etc.
And there is a throw designation.
ST, DT for single pole; Double thow
So, you get things like SPST and DPDT
That isn't enough to specify a switch.
DPDT (on)-off-on is a way to specify that one side is momentary (), there is a center position, and the other side is on
DPDT on-none-on does not have a center position,
Both switches would have 6 wires.
There is one switch that doesn't follow these rules and it;s used in control panels.
and that's the SPST - 1NO/1NC switch
An SPST switch normally has 3 wires. a 1NO/1NC switch has 4 wires.
Two contacts break before another two contacts make.
The NO and NC mean Normally Open and Normally Closed. This nomenclature is seen with relays as well. You might see "1 NO AUX contact"
or One Normally Open Auxiliary Contact
Furthermore, some switches are "make-before-break (shorting) or "break-before-make (non-shorting). The former was used in audio applications to eliminate pops. So, for an instant, all of the switch contacts are shorted.
Switches and relays can be called "Definite purpose". Examples are "A reversing switch" and a "Lighting Contactor".
I'll stop here and let stuff digest a bit. I did "mention" some similarities between relays and switches.
An example of a switch that's really common in a lot of households is the reversing switch on a ceiling fan. It is not supposed to operated when the fan is on.