When you read a magazine for affluent, professional adults, you are a target market. When you watch a cartoon on cable and think the cartoon is perfect for girls and not that great for boys, you're recognizing the target market. When you see an advertisement for detergent during the afternoon talk shows or soap operas, you are a target market.
Advertisers have known for years that it's easier to sell to a small segment of the population, rather than to everyone in general. If you're a company who has invented computer screen that helps improve visibility for people with decreased vision, you won't want to sell your product to young, active adults, nor the general population. Your target market is probably the elderly. This is what a target market is.
A target market is the specific segment of the population that advertisers (and all media producers) aim their messages at. For example, if you are reading visiting this website, you are probably an educator, a parent, or a student. You are probably not a plumber looking for plumbing supplies.
As funny as it sounds, it's true: all media consumers (which is everyone) are part of at least several target markets. If you're a male or female in your twenties, you are a target market for radio stations, colleges, inexpensive cars, credit cards obtainable with low credit scores, the latest computer gadget, and definitely a lot more.
Advertisers know about your target market's lifestyle, and can tailor products and services to fit your needs and desires. Sometimes advertisers create a desire where one didn't previously exist.
By knowing that you're a target market, you can become a smarter consumer of media.