Firstly there are lots of ways and techniques used to slow a plane down.
On the ground, planes are equipped with toe brakes. These are located above the rudder pedals and are depressed using your toes. Other planes may have brakes like a handbrake on a car. Larger planes use "reverse thrust" where the thrust which normally flows towards the back of the plane is deflected forward thereby pushing against the forward motion of the plane and slowing it down.
In the air, the principle is much the same as a car, where one is to reduce the power of the engine in order to slow down. That is, you pull back on the throttle. The only difference to a car is that in a plane when you reduce power most planes will nose down slightly and start flying downward, whereby due to gravity the plane will then try to speed up. To combat this what the pilot has to do is reduce the power by pulling back on the throttle and either maintain nose level or pull back more on the stick to raise the nose as well.
Of course you can pull back on the stick without removing power and the plane will then enter into a climb at a reduced speed.
Larger planes are equipped with spoilers and speed brakes that extend from the wing and basically spoil the lift that the wing is producing. Thus, the pilots can keep the nose up without gaining altitude and thus slow down the plane. Even, on landing the pilot will be able to keep the nose of the plane off the ground for as long as possible to give the maximum air resistance against the body of the aircraft to slow it down faster on the landing roll.
Other techniques to lose height and slow a plane down include using opposing aileron and rudder. This opposing controls results in the plane wanting to drop like a brick, if you then pull back on the stick to try to maintain height the plane will slow down.