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 takes all forms - print, online, podcasts, YouTube videos, radio shows, images - any format that can convey information can convey disinformation.
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.
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."
Publications by advocacy organizations or think tanks: organizations such as The Sierra Club, Moveon.org, the National Rifle Association, can produce useful materials but they should be understood to represent a particular point of view. It would be wise to seek other viewpoints for a more balanced understanding of the issues.
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
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
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