The five stages in the product life cycle are product development, introduction, growth, maturity, and decline.
The product development phase is the phase in which a company has a new idea for a product. For example, a fast moving consumer goods (FMCG) company comes up with a new idea for a weight loss supplement in the form of a milkshake. During this phase, the company is not making any money, because the product is not actually on the market yet.
In the introduction phase, the new milkshake is introduced to the market, probably with a big marketing campaign. While sales of the product slowly start to pick up, the company is still not making a profit, as they are recuperating from their start-up costs.
In the growth phase, the milkshake starts to get popular. People who want to lose weight are seeing results from using this product, and the company starts to turn a significant product.
In the maturity phase, profits are still good, but are not growing as much as they were in the growth phase, because most people who are interested in this product have already been aware of it and using it since the growth phase. In other words, there are fewer new customers.
In the decline phase, sales figures start to drop and less profit is seen. There could be a number of reasons for this. It's possible that a competitor has brought in a new product that has grabbed the attention of the target market, or a perception could develop in the market that the weight loss gains gleaned from this product are temporary.