Periodisation emphasises different aspects of training, in successive cycles over a period of weeks or even months.
All Elite athletes tend to adopt a periodisation approach to their training. First introduced by Arthur Lydiard, put simply Lydiard said athletes need to train on endurance - long runs, before moving onto their strength work, then moving onto faster paced Anaerobic sessions nearer the time of the race.
Although we have periodisation, generally speaking all aspects of training are evident within a programme all year.
Periodisation is just emphasising a specific element, such as speed or strength, during a cycle of work. All aspects of a training programme may be emphasised with a multi tier system of training, such as was introduced by Frank Horwill.