Let’s take a closer look at each.
Advertorial, also known as "sponsored content," is a piece of editorial material where the editor has commercial interest. So, when a brand pays a newspaper to publish an article that paints its products in a positive light, that's an advertorial. Each publisher has their own advertorial guidelines - some allow commercial content to mimic editorial without attribution, others are more explicit. Well researched advertorials that provide deep, engaging content can perform very well for advertisers and pay a premium to the publisher. However, as any good content it requires time to produce and is therefore hard to scale. Poorly targeted advertorials without attribution can be perceived as a deceptive practice by the consumer, so brands and publishers need be careful to project a positive image.
Content Marketing is the practice of using content to drive attention to other content. The difference from an advertorial here is that the buyer is not necessarily making an offer to purchase to the consumer. For example, content marketing is frequently used by news sites to buy traffic from other news sites. The benefit to the buying publisher is a potential increase in the additional ad revenue driven by traffic they "bought" from other sites similar to theirs. The benefit to the selling publisher is incremental revenue from traffic that was leaving their site anyways. This strategy allows publishers to capitalize on popular pages about recent events, seasonal travel destinations, etc. A larger publisher that charges a higher CPM on its site can acquire traffic from smaller sites cheaply and earn revenue on the spread. The added bonus here is that both sides offer pure editorial content, so the user is continuously engaged, and not deceived. Brands can also utilize content marketing very effectively by creating educational content for their consumers, and using content marketing to drive traffic to their blog and social media pages. HubSpot calls this practice "Inbound Marketing." Thanks to offerings like OutBrain and Zemanta, and recent advances in semantic analysis, content marketing can be automated at scale.
Native Advertising is the newest term here, and is yet to obtain a universally accepted definition. In my view, it is defined as content that seamlessly blends into the context, design, and functionality of every page that it appears on, while being overt about its sales intention. The goal of native advertising is not to conceal an offer to purchase - it's to make this offer so relevant that each user will be compelled to follow through and explore it. By definition, native ads must be as much user-driven as the rest of the content on the page. Native advertising has actually existed for a while before having a term coined for it: Anthony Young of AdAge brings up Google AdWords as an example. Facebook sponsored posts is another one. The key that differentiates Native from traditional advertorials and content marketing is that native doesn't need to "mimic" content. It needs to be a call to action of content, whether it is sponsored or not. A variety of native advertising platforms exist, and each offers unique pricing, solutions and scale. This year, publishers, brands and agencies will need to evaluate emerging native advertising opportunities on their individual merits.