What do flaps do?

The 747's (pre-747-800) beautiful triple-slotted flaps. These have DRAMATIC effects.

The old 737's deployed slotted flaps.

This diagram is a good basic summary of wing components. Of course, every plane is different, but this can act a general guide. When you're actually in a plane, it often easier to figure out what everything is based on what movements its making when.

As you can see, normally on larger airliners there are two sets of flaps, inboard and outboard. On smaller planes, the wings are too short to have two sets of flaps be practical. Flaps are usually only used during landing and takeoff and have two functions. When lowered, flaps increase the wing area, thus increasing the amount of lift the wing is generating. Another way of looking at it is that flaps decrease the stall speed of an aircraft, allowing it to fly more slowly without dropping out of the sky. The second force resultant of lowering flaps is an increase in drag. Since flaps drop down (at varying angles depending on the aircraft) from the wing, they create lots of drag, helping the aircraft slow down. Flaps are especially useful as they allow the pilot to decrease the angle-of-attack of the aircraft without increasing the airspeed. (Angle-of-attack is a fancy pilot word for pitch. When a pilot lowers the nose, he is decreasing the angle-of-attack. Normally this would increase the airspeed. But with flaps down, the aircraft will accelerate much more slowly as the flaps are generating tons of drag.)

Flaps have variable settings, measured in degrees that will determine how dramatic their effects are. I will use my little Cessna 152 as an example:

0 degrees: Flaps are not deployed and are thus not generating any additional aerodynamic forces.

10 degrees: Flaps are 10 degrees down from the chord (the line from the leading edge to the trailing edge of the wing). At this setting, they are sensibly (as in I notice) only generating additional lift. Typically I will use this when I want takeoff in a slightly shorter distance.

20 degrees: The flaps start generating some drag, meaning that if during flight I were to lower them straight through to 20 degrees, you would feel some negative acceleration. Otherwise yet more lift.

30 degrees: This is the normal landing setting. The plane significantly decelerates and you would be thrown forward into your seat belt if they were deployed to 30 degrees midflight. (As a side note, flaps can only be deployed below a certain speed (Vfe), usually well below cruising speed, otherwise the pilot risks damaging both the flaps and the wing.) Here the lift and drag is very high. Normally, in my Cessna, 30 degrees of flaps was the setting that would allow me to fly the slowest without stalling.

40 degrees: I rarely use this puppy. At 40 degrees, far more drag is being generated than lift. This is used to lower your altitude as fast as possible while staying within a certain airspeed envelope. I never would land with 40 degrees unless absolutely necessary, as the flaps are at such a steep angle that the wind can catch them like a sail at low speeds and balk my landing.

The summary of what each setting is used for basically demonstrates how flaps are used in general. Low flap settings are used for takeoff to increase lift for a given speed, thus shorting the take-off distance, and to slightly help slow the aircraft during landing. High flaps settings are using during landing to allow the plane the fly much more slowly while not risking a stall, as well as aiding the pilot in slowing the aircraft down during approach.

There are many different types of flaps, each with their own advantages and disadvantages. But instead of overwhelming you with all that unnecessary knowledge, I'll just show a little diagram of what they look like and end there. My boring little Cessna had plain flaps of course.