The term "fake news" most literally applies to completely fabricated stories, but recently has come to refer to a broader continuum of news sources. It's important to note that many (arguably, most) news sources have some kind of explicit or implicit perspective, but that this doesn't mean that those sources automatically qualify as fake news. That's why it is extremely important to be able to assess the quality of content - ultimately it is up to you to make sure that your information is good.
It can't be verified
A fake article may or may not have links tracing its sources, and if it does, they may not lead to articles outside of the site's domain or may not contain information pertinent to the topic of the article. It might refer to a retracted study, be based on a naturalistic fallacy, or not refer to a study at all.
Fake news appeals to emotion
Fake news plays on your feelings - it makes you angry or happy or scared. This is intentional - an emotional response makes you more likely to believe it and less likely to fact-check the article.
Authors usually aren't experts
They might have no credentials at all, or inappropriate credentials from the topic, they might not be professional journalists, and they might even be paid trolls.
It can't be found anywhere else
If you look up the topic of a fake news articles, you might not find any other news outlet (real or not) reporting on the topic.
Fake news comes from fake sites
Did your article come from abcnews.com.co? That's a website designed to trick you into thinking you're on ABC News' website. Did you find an article on a natural treatment from a random website not affiliated with a university, library, research institution, or other reliable source? Proceed with caution - it might be fake news.