There are many ways in which you can classify an audience. The simple demographics to classify an audience is their location, gender, race, sexuality, religion, age, and occupation. 
An audience is commonly classified by class, which is normally determined by wage and occupation. This way, companies can aim their product at specific groups of people. 

Companies also look at physiographics to define and categorise an audience. This looks more at their personality and interests, as well as to work out what they are more inclined to want to buy and how advertising would effect them and make them more likely to buy a product.

Now there is The LifeMatrix model. This categorises an audience into 10 different, more specific types. This now is not just about class, but now more about age, interests, and attitudes. Using the LifeMatrix model makes it easier for a company to aim a product at an specific audience. ​