Google announced that as of yesterday, interest categories are now available to all AdWords advertisers. Interest categories are a good way of exposing your content campaigns to more users within the display network. We always recommend that client set up a re-marketing in order to get more data and understand the creative performance
How interest categories work? According to Google “the system looks at the types of pages a user visits, taking into account how recently and frequently those pages have been visited, and then associates their browser with relevant interest categories. Using these categories, you can show ads to the people most likely to purchase your products or services, and you can reach them across all types of sites in the Google Display Network in addition to contextually relevant sites.”
There are over 1,000 interest categories from very niche to very broad interests/themes. Once more Google is helping advertisers to use the Google Display Network.