There are many types of switches. Here are the basics and how they boil down to the simple switch in most ways:
Below are some switch packaging that may not be familiar at first, but electrically they are the same as the simple switches discussed above.
Arcade Pushbutton Switches
Adafruit carries a wide selection of this type of pushbutton switch. They come in various sizes and colors.
There are also arcade switches with an LED inside to indicate switch status - this is usually a second set of terminal tabs on the bottom of the switch and you have to hook up the appropriate leads to power the LED when you want it to glow.
Slide switches are activated by moving the button to the left or right. The one to the left is a single pole, double throw and is made to fit the pin spacing in breadboards.
Other switches on the market, especially older ones may have multiple positions of multiple poles. If unsure, use a multimeter to get a feel for which contacts connect when in a certain position.
Tactile switches are most often used on circuit boards and breadboards. They are inexpensive and easy to use (as long as the contacts are connected correctly, see the Hook Up page for help on that).
Toggle switches are good for a nice, tactile feel and a visual if the switch is in one position or another. They can come in Single Pole Single Throw, Single Pole Double Throw, and Double Pole Double Throw configurations.
Some toggle switches have a guard on top to prevent them from accidentally being activated by a bump of a panel. Others may have extra leads for a lighted connection.
A micro switch is another name for a small switch that is usually triggered by some movement onto it's switching mechanism. The switch may have a lever to help it trigger.
A foot switch is just another switch, but it is triggered by one's foot. Often single pole single throw or single pole double throw.
Adafruit sells several tilt sensors. They are just a switch that activates not by push but by tilt. You can read them the same way as the switches discussed earlier.
Vibration Switches look similar to tilt sensors. The one to the left has been opened to reveal the switch will activate when bounced around, not tipped, due to the spring.
Magnetic Contact Switch
At left is a single pole single throw simple magnetic contact switch that activates when a magnet gets near it.
Mag switches can come in other configurations such as single pole, double throw, and double pole double throw from other manufacturers.
There are many, many more types of switches you may encounter. If you just remember the basics, it's very much the same: use a multimeter, check the terminals before and after switching, jot things down on paper to remember.
Use a manufacturer's diagram if one is available.
For switches with built-in lights, you'll find contacts that do not change when switched. Those most likely are the lighting terminals. If you can, try to find the manufacturer's details on the switch so you know what is the switch and what is the lighting contacts. The information on the lighting will tell you polarity (if any) and if there are any other components inside like an LED current limiting resistor. This will help you design your project's circuitry.
Some switches have multiple, independent switches that are activated by the same action.
You can see the multiple contacts in the different types of switches above. Usually the center pole is in the center of the pin group if rectangular. Switch one way it connects one set of contacts, the other way connects separate contacts (there may or may not be a "center" where there is no contact).
Use a multimeter (or a battery + resistor + LED) to see which terminals are connected when in various switch positions. Draw a diagram on a scrap of paper (or your Maker notebook) to remember what connects to what.
Double pole, Double Throw Switch
Sometimes there is a center position where the switch isn't connected through to any pole, called "center off".
You can see these switches and others on Wikipedia. If you only need a single pole single throw but have one of the switches with additional terminals, you can safely ignore them. Or you can wire something so in one direction you trigger a green light, in the other a red light as an example.
You will see many projects using capacitive touch as a switch. Capacitive touch is not a switch in the sense this guide is defining a switch as there is no mechanical motion involved.
The Adafruit Learning System has many guides on the use of Capacitive touch in various circuits. If the switch you are contemplating is capacitive touch instead of mechanical, search the Adafruit Learning System for more information.