Yet another question answered in the Welcome Mat. Here's an excerpt:
Q: How do I choose a switch?
A: The "reverse clickie" is very commonly available in inexpensive and general use household and outdoor flashlights. It has a button which must be fully pressed until it clicks and then fully released in order to get light. Once the light is on, a soft press will momentarily turn the light off, and once the button is released, the light will reactivate. This is a very inexpensive (and hence popular) switch type, but it is not suited for signaling or momentary operation.
A "forward clickie," however, can be gently pressed from the Off position to momentarily activate the light, and it will turn off once the button is released. The user can also push the button until it clicks to keep the light on. This is available in some Maglite, Streamlight, Inova, and Surefire models.
Another type of switch is the Lock-Out Tailcap, or LOTC. Such a switch has no clicking mechanism. It can be "locked out" (prevented from turning on, useful for storage and transport) by unscrewing the tailcap to some degree. If the tailcap is tightened enough, pressing down on it turns the light on until it is released. If it is tightened further, the light turns on and stays on. This type of switch is most common in tactical Surefire models. Some lights have a switch similar to this that cannot be locked out, such as Streamlight's TL-2 and TL-3. They are still suited to tactical use.
The simplest switch is the twisty. In such a flashlight, two components (usually the body and head) are tightened together to close a circuit and turn the light on. These are highly valued for their perceived reliability, but thread quality and frequency of use are a large factor in the system's longevity.
There are several other types of specialty switches, including magnetic reed switches, Hall effect switches, membrane-covered electronic contact switches, and more, but they are best discussed in conjunction with the lights that use them.