The product lifecycle is a very simple graphical model which explains the life of any given product:
The life of all products should follow the same pattern, this is because consumer tastes change, and competitors are always entering the marketplace. Very few products remain at the maturity level, products which remain at the maturity level are things like 'Kit-Kats', 'Kellogg's', 'Mars Bars' which after heavy advertising maintain their market position.
The product lifecycle model feeds into the Boston Matrix model to help us analyse the market further.