Generally flaps are use only on the inboard section of the wing. The reason is that while flaps increase a wing sections maximum coefficient of lift, that maximum is achieved at a LOWER angle of attack. A wing section that, when clean, may stall at 16 deg AOA may now stall at 14 degs with flaps; however, at 14 deg with flaps it will be producing more lift than when at 16 deg clean.

What this means in practical purposes is that while you may achieve more lift for slower flight with outboard flaps, you run the risk of stalling the outboard section of the wing first and thereby loose roll control (the dreaded tip stall). You would not want a wing that stalls the tip section 2 deg before the root section.

As a side note, slots and slats are just the opposite. That is, they increase the maximum lift (like flaps) but they INCREASE the stalling AOA. That is why you generally see them near the tips of a wing.

Bill Vail