In order to change from one state to another, a switch must be actuated. That is, some sort of physical action must be performed to “flip” the switch’s state. The actuation-method of a switch is one of its more defining characteristics.
Switch actuation can come from pushing, sliding, rocking, rotating, throwing, pulling, key-turning, heating, magnetizing, kicking, snapping, licking,...any physical interaction which can cause the mechanical linkages inside the switch to come into, or go out of, contact.
Momentary vs. Maintained
All switches fall into one of two distinct categories: momentary or maintained.
Maintained switches -- like the light switches on your wall -- stay in one state until actuated into a new one, and then remain in that state until acted upon once again. These switches might also be called toggle or ON/OFF switches.
Momentary switches only remain active as long as they’re actuated. If they're not being actuated, they remain in their “off” state. You’ve probably got a momentary switch (or 50) right in front of you...keys on a keyboard!
Semantic alert! Most of the switches we refer to as "buttons" fall in the momentary category. Activating a button usually means pressing down on it in some manner, which just feels like a momentary control. There are such things as a maintained button, but for this tutorial when we slip and talk about "buttons", think “momentary push-down switch”.
As with most components, the termination style of a switch always comes down to either surface mount (SMD) or through-hole (PTH). Through-hole switches are usually larger in size. Some might be designed to fit in a breadboard for easy prototyping.
SMD switches are smaller than their PTH counterparts. They sit flat, on top of a PCB. SMD switches usually require a gentle touch, they’re not built to sustain as much switching force as a through-hole switch.
Panel mount switches -- designed to sit outside an enclosure -- are a popular mounting style as well. It’s hard to flip a switch when it’s hidden inside an enclosure. Panel mount switches come in all sorts of termination styles: PTH, SMD, or heavy-duty solder lugs for soldering to wires.
One more important switch characteristic, which really deserves a page of its own, is the internal circuit arrangement of a switch. Are you looking for an SPST? DPST? 4PDT? What-P-what-now?