The Four Basic Forces of Flight

For an aircraft to maintain straight and level flight it must match the forces of nature (drag and weight) with what it is able to control (thrust and lift).

For an aircraft to increase its speed it must accelerate. To accelerate, the force pushing it along (thrust) must be greater than the drag (air resistance) it faces. Doing so will naturally increase the drag as it is hitting more air particles per second, but it's not enough to stop the acceleration.

If the thrust was equal to the drag then the speed (whatever speed it is) would remain constant. Be it 0 or 100 KM/H. If the thrust was less than the drag, however, the aircraft would slow down (decelerate).

The same can be said for lift and weight. If lift is greater than the weight the aircraft rises, visa-versa it falls, and if they are equal the aircraft remains at its current altitude.

An aircraft can utilise these laws of nature (like it wasn't already in order to fly) by raising air brakes in order to increase the drag on the aircraft (larger surface area, and less aerodynamic) and help it slow down. Lowering flaps increases lift, making it easier to take off as lift is being generated more efficiently.

Below is a diagram from NASA, explaining the whole thing.

NASA's quite good at making things fly, so I'd trust them on this one.