Marketing is really an art nowadays. Making a good marketing campaign is getting harder and harder because market is so filled with products that people are really tired of all commercials. Because of that, mass marketing does not work anymore. Not so many years ago, companies were buying television ads, transparent, jumbo rolls and other similar marketing techniques. Today, this is considered a waste of money, because it has been proven so many times, that marketing through market segmentation is so much more efficient. I will shortly describe what does a market segmentation theory says about marketing.
The first and most important point of market segmentation theory is, that there is no point in spending money for marketing of your product to certain people, if these people will not buy the product. For example, you will not market skirts to boys in USA. A very simple market segmentation in that case would be women segment of the market.
You need to decide who is your target group and they try hard to promote/market your product to that particular group. This is market segmentation. Why is this good? Because if you have enough money to promote your product to 100 people for example, it would be just stupid to promote it to people who will cannot buy. You would gain many more sales if you would market it only to potential buyers.
So to summaries, market segmentation theory is all about separating the market into smaller groups of consumers and then marketing your product only to the group of people that are your potential buyers.