Simply put, a periodized program is one divided into periods or phases (cycles) where each period has a different objective and as such different physical requirements. They are highly planned and scheduled processes.
Some periods demand that you work hard, whilst other periods pursue gentler goals.
As each period has different outcomes scheduled, the intensity (how hard you work) and the volume (how long you work) at each element varies. These elements are controlled in order to achieve the desired outcome of the particular period.
This process keeps your body "confused" and therefore in a continuous state of adaptation, maximising the benefit of your training.
Periodized programs even allow for periods of regeneration by "unloading", greatly reducing the demand you put on yourself in order to allow for maximal adaptation to occur, allowing for better performances than usual.This is the process most modern athletes use, and is often referred to as a "taper".
