What exatly are they? None of my planes have them, but for curiosity's sake why do some planes have them and others don't?

Flaps change the camber or curvative of the inboard wing platform. They are actuated at low airspeeds to provide both additional lift and drag.

Some sport planes like the Stick series from H9 use them for a more aerobatic flyer.....they are a lot of fun to play with....

Jerry

Flaps are installed to allow one to fly slower. They decrease the stall speed by creating additional lift.

Also in addition to everyone is saying with flaps deployed hense creating more lift. Some of the heavier warbirds use flaps on take off to create the extra lift to get them in the air.

You are all right to a point. Flaps do increase lift, but beyond a certain degree angle, they increase more in drag. That is why you are able to come down at a steeper angle of attack, and not increase airspeed. Models are all overpowered, so full flaps to them WILL increase lift,.....with a hefty addition of power!!

I've got a ducted fan sport model called a Renegade. I've always landed it with flaps down, but at first it only had around 25 or 30 degrees throw on them. I had a lot of trouble with overshooting the runway, as the plane's slick and relatively heavy (though not so heavy for a jet). With closer to 45 degrees flap my problem was essentially solved; now I can make a steeper final and, if it's coming up short, just bump the throttle up until it gets where I want it.

I don't think that the flaps slow the stall speed very noticeably; in fact possibly the reverse. For myself, I'd rather have a plane that can make a steep descent rather than fly very slowly. The steep descent means that you don't have to gauge your airspeed or altitude as accurately when you turn onto final, and so you can play it safe by keeping it well above stall speed.

That plane sold me on the usefulness of flaps for relatively low drag models, and in particular on the value of real, separate flaps with large throws as opposed to flaperons which usually have much less throw.