The TARGET MARKET is the area you wish to enter your product in. For example, role-playing games, action movies, etcetera. The TARGET AUDIENCE is the demographic you're marketing the product to and, as such, are the people you aspire to gain sales from. These are typically in the form of income bands, age groups, interests, etcetera. The target audience can be decided upon through MARKET RESEARCH.
PEGI - Pan European Game Information (pegi.info) - provides information in the form of labels on products describing the age group (3, 7, 12, 16, and 18) and content (sex, fear, language, gambling, discrimination, drugs, violence, and in-game purchases). Their labels are usually represented as individual stamps.
The US instead has ESRB - Entertainment Software Rating Board (esrb.org) - which has rating categories (E, E10+, T, M, AO, and RP), content descriptors (language, violence, suggestive themes, etcetera), and interactive elements (user interaction, in-game purchases, etcetera). Compared to the individual stamps PEGI uses, ESRB has all information in a single place, allowing for browsing the content without having to pick up the product unless they want more details.
These organisations, alongside others for different forms of content, exist to help consumers determine if a product is right for them or the person they're buying it for, as well as to keep developers in line by showing what content will get them specific ratings.