In basic terms:
The combination of pitch attitude setting and power setting will determine whether the aircraft flies level or climbs or descends, and also the speed at which it flies. To make the aircraft achieve a desired vertical flight path and speed the pilot must select the appropriate pitch attitude and power setting. Only one combination will give the correct result. The pitch attitude chosen by the pilot is sometimes used to control the vertical flight path of the aircraft and sometimes to control its speed.
When a defined vertical flight path is the intention, pitch attitude controls vertical flight path and power controls speed. Examples: straight and level, approach (including ILS) and landing flare.
When power setting is fixed, pitch attitude controls speed. Examples: climbing and descending.
These methods are valid for all aircraft, from Piper Cub to A380. For analysis in greater depth, look at