Flaps are on the trailing edge of the main wing. Usually, the center is a flap and the outboard is an aileron. Some people leave the whole surface as one and use it as ailerons, and couple them so they both go down the same amount. That is called flaperons.

They are good in some cases for slowing the plane. I have used flaperons and they are good for some planes, bad for others. My glider has flaps that go out about 1/2 way to the tip. The ailerons go the rest of the way. That plane can drop the flap almost 90 degrees and the flaps are very effective. I can also raise them some to give the plane lots of speed in the air.

There are lots of ways to do the flaps, just depends on what the plane is and what you want them to do. Approx the first 20 degrees will provide some lift, any more is drag for slowing the airplane.