Wikipedia defines fake news as: “Fake news is a type of hoax or deliberate spread of misinformation in social media or traditional news media with the intent to mislead in order to gain financially or politically.”
“Fake news websites ... deliberately publish hoaxes, propaganda, and disinformation — using social media to drive web traffic and amplify their effect.”
Fake news takes all forms - television, print, online, podcasts, YouTube videos, radio shows, images - any format that can convey information can convey disinformation.
Tip: If the URL ends with ".co" such as ABCnews.com.co or washingtonpost.com.co then it is probably a fake news site.
These types of web pages may be legitimate but they should not be mistaken for impartial news sources:
Opinion pieces/editorials - these may be written by journalists or experts, but they should be clearly marked as opinion pieces and not mistaken for an impartial news report.
Parody/satire sites, including The Onion and the New Yorker's Borowitz Report; their purpose is to entertain and perhaps to persuade, but they should not be mistaken for news sources. See a list of some satirical sites here.
Native advertising or "sponsored content" - their purpose is to sell, not to inform. Wikipedia defines native advertising as "a type of advertising, mostly online, that matches the form and function of the platform upon which it appears. In many cases, it manifests as either an article or video, produced by an advertiser with the specific intent to promote a product, while matching the form and style which would otherwise be seen in the work of the platform's editorial staff." See some examples here.
Press releases - public relations pieces from a company or organization; they often are marked "For immediate release" or say, "Press Releas." They are often quoted or published in full by legitimate news sites, usually with the statement, "according to the press release."
Publications by advocacy organizations or think tanks: organizations such as The Sierra Club, Moveon.org, the National Rifle Association and The American Enterprise Institute, can produce useful materials but they should be understood to represent a particular point of view. To learn which particular point of view they represent it is helpful to learn who funds the organization or think tank. It would be wise to seek other viewpoints for a more balanced understanding of the issues.
How to Spot Fake News
How to Spot Fake News FactCheck.org November 18, 2017
'Fake News' Is Real. Here’s How To Know If You’re Reading It' Huffington Post February 1, 2017
Creation/Spread of Fake News
How the Internet is Loosening Our Grip on the Truth New York Times November 2, 2016
How Fake News Goes Viral: A Case Study New York Times November 20, 2016
Facebook fake news writer: 'I think Donald Trump is in the White House because of me' Washington Post November 17, 2016
We Tracked Down a Fake-News Creator In the Suburbs. Here's What We Learned. NPR November 23, 2016
When Fake News Stories Make Real News Headlines ABC News November 29, 2016
The city getting rich from fake news. BBC News December 5, 2016
Fake news, real violence: 'Pizzagate' and the consequences of an Internet echo chamber CNN December 6, 2016
From Headline to Photograph, a Fake News Masterpiece New York Times January 18, 2017
Effect of Fake News on Readers
Students Have "Dismaying' Inability To Tell Fake News from Real, Study Finds NPR November 23, 2016
Liberals Believe Fake News Too Slate November 29, 2016
The very real consequences of fake news stories and why your brain can't ignore them PBS December 5, 2016
As Fake News Spreads Lies, More Readers Shrug at the Truth New York Times December 6, 2016
Most Americans Who See Fake News Believe It, New Survey Says BuzzFeed News December 6, 2016
Many Americans Believe Fake News is Sowing Confusion Pew Research Center December 15, 2016
Why Nobody Cares the President Is Lying New York Times February 4, 2017
Proposed Solutions
Google and Facebook Take Aim at Fake News Sites New York Times November 14, 2016
How to Stop the Spread of Fake News New York Times November 22, 2016 (opinion pages)
Misinformation on social media: Can technology save us? The Conversation November 27, 2016 (opinion piece)
How Data and Information Literacy Could End Fake News Forbes December 11, 2016 (opinion piece)
Finder's Guide to Facts NPR December 11, 2016