Flashlight Basics
A flashlight (or "torch" for some English speakers) is a handheld light source powered by batteries.
Light Source
Traditional flashlights have an incandescent bulb and are powered by a couple of alkaline batteries. They are On or Off. But in the past ten years or so LED flashlights have made enormous leaps in performance offering more light, longer bulb life, and greater efficiency, meaning you can use smaller batteries or that your batteries will last longer. Incandescent light bulbs have made progress too, with xenon and halogen bulbs available.
Reflectors and Lenses
Most flashlights include a reflector around the light source to gather the light and focus it out the front. Reflectors have a parabolic shape which can take light rays emitted from a light source in the parabola's focal point and send the reflected rays all in the same direction (rays that go out the front without hitting the reflector will provide some spill). A bigger, deeper reflector is able to focus the light into a smaller hotspot and offer more throw and less spill. Smaller lights are easier to carry in a pocket but will generally have more floody light. Mag Instrument's Maglites have a reflector that moves with respect to the light source allowing a variable focus that can yield a wider flood or a tighter spot of light.
Usually there is a flat glass or plastic lens over the reflector. Better lights have glass lenses with anti-reflective coatings that allow more of the light to pass through the lens rather than bounce back into the light (even better than plain glass). But some lights have a curvilinear lens that concentrates the light similar to a reflector. And still other lights let you move this lens and go from flood to throw, similar to Maglites. See lenses for more options.
Batteries
Powering the light are batteries which are usually stored in the body of the flashlight with the positive end towards the front. A wide variety of batteries are used. Disposable batteries, or "primaries," include heavy duty, alkaline, and lithium batteries. Rechargeable batteries include NiMH and lithium-ion batteries among others, offering "guilt-free lumens" and often greater performance than primaries.
The Switch
Current from the batteries is controlled by the power switch. While traditional lights have this on the side, smaller lights usually put the switch on the tail of the light, so that it can be activated by your thumb. A tail clicky, when turned to On, completes a connection between the bottom, negative end of the batteries and the body of the flashlight where the electricity travels up to the head of the flashlight causing a difference in potential that is used to power the light source. Clickies can be "forward" (allowing momentary On when the switch is halfway pressed) or "reverse" (the light does not turn on until the switch clicks into place, but then allows momentary off). Some lights feature electronic switches which work a little differently by sending signals to a microcontroller. There are also twisty switches where you twist the tail or head of the light to complete the electrical connection. Another type of switch used on some Surefire and 4Sevens Maelstrom lights doesn't click at all. If the tailcap is unscrewed the light won't come on, if it is partially tightened you can use the button for momentary on, and if the tail is fully tightened the light stays on. This switch is good for high current uses since the switch is very simple in construction.
Driver
In the head of a LED light there might be a driver which regulates the voltage from the batteries to some value optimal for the light source. For instance many LEDs need a voltage of about 3.6 volts, so if you only have one AA battery (1.2 to 1.5 volts), the driver will need to boost the voltage to something the LED can use. The driver also can control different brightness levels to the light and special modes for SOS or strobe functions. Incandescent flashlights are often driven directly by the batteries without a driver.
More Options
Flashlights themselves can take different forms, including head lamps, and can be made of plastic, aluminum, stainless steel, or titanium (see materials). There are many different kinds of light sources and LEDs that can be used too.