There are multiple ways/format to buy Inventory (advertising space) on a publisher website. One of them would be Native Advertising.
What is Native Advertisement ?
Generally when you find a paid content on a publisher website which is not in Image/Banner/Gif Formats and doesn’t appear like an advertisement! The look and feel of such content and even the message is matched with publisher content. Still not sure? See this : more examples
Remember those Sponsored content during recent Parliament elections?