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A spoiler, named because they sometimes "spoil" a fan's enjoyment of the canon, is information (usually plot-related) about upcoming or recently-released serial source information.
History of the Use of the Term
One of the first print uses of the terms was in the April 1971 issue of National Lampoon. An article entitled "Spoilers," by Doug Kenney, lists spoilers for famous films and movies.
Another early use was mentioned in 1980 when a fan chides another for giving away the ending to a story and writes: "At the very least, I suggest those who plan to review fanworks, take the time to read the columns in pro sf zines. Inevitably, you will cross the path of Spider Robinson (reviewing for Analog right now) and his invarying use of the simple 'WARNING: I AM ABOUT TO COMMIT A SPOILER. IF YOU DO NOT WANT TO KNOW THE END OF THIS STORY, SKIP THE REST OF THIS PARAGRAPH.'" 
Definition and Types
What is or is not a spoiler is often a matter of debate. Creator statements, such as from interviews, are not generally considered to be spoilers unless they refer to material that will be explicitly featured in canon later on.
Some fans also consider information about what actors will appear in what episodes of a tv series to be a spoiler; for instance, the surprise return of a long-lost character could be spoiled if fans know that the actor who plays that character is returning to the show. These are often referred to as "casting spoilers."
"Mood spoilers" are another type of spoilers. On Livejournal and other journalling sites, the author of a post can often specify what "mood" she is currently in, such as "elated" or "depressed." Some fans believe that even in the absence of all other spoilery information, indicating one's mood in this way can be spoilery. A mood spoiler can also be found in a title or an lj-cut; for instance, if a fan who was well-known to be a die-hard shipper of a certain pairing made a post titled "OMGSQUEE BEST EPISODE EVER," it could possibly be inferred that there had been positive developments regarding her OTP. Other fans have stated that it's not so much a single mood spoiler that is the problem, but that an entire flist full of various mood spoilers makes it somewhat impossible to not draw inferences about the quality of, or new developments in, new canon.
The term spoiler is sometimes used to refer to a fan who shares spoilers. This term describes their fannish activity and doesn't necessarily carry negative connotations. In some fandoms, spoiler communities (communities of fans seeking out spoilers) are common and trusted spoilers sometimes achieve BNF status within these communities. An associated term is foiler, which refers to fake spoilers or fans who intentionally share fake spoilers.
Many communities require spoiler warnings or outright ban spoilers in posts. Spoilery information (for discussion) generally has an expiration date, after which it is acceptable to discuss without warning other users. For example, an expiration date could be one week after the US or UK air date, for a television series, or one month after the release date of a comic book. Since fans in different geographical regions may not be able to view episodes for weeks, months, or even years after their original air dates, some fans and some fandoms have longer statutes of limitations.
Fandoms that actively cultivate new fans, such as Farscape fandom, are often careful to welcome viewers watching the show for the first time without spoiling them for future plot points.
It is common to give a spoiler warning in the header of a fan fiction story if it contains numerous references to a specific episode or book. This is far more common in the immediate aftermath of the episode, when many fans either may not have had a chance to view the new canon, or may be specifically seeking out fan fiction that deals with evetns in the new canon.
One common tactic for encoding spoilers, often used on Usenet newsgroups, was rot13. But by the time fandom had taken up residence on mailing lists, it was more common to use the word "Spoiler" in the subject line and to provide "spoiler space" -- multiple lines of space before the spoilers appeared. On blogs and Livejournal-type services, spoilers are more often put "below the cut", requiring a reader to deliberately click a link to see the spoiler.
Some fans also use HTML to hide spoilers; for instance, putting spoilers in white text on a white background. This requires the reader to higlight the text in order to be able to view the spoilers; however, it is not always implemented correctly, sometimes resulting in unintentional spoiling.  In addition, this formatting may not be accessible to, or may not work to obscure the spoiler for, all readers, such as those who browse the internet using screen-readers.A 2010 example of the tightrope some fandoms ask their fans walk is illustrated by this correction by a Live Journal Professionals community mod to a new fan who'd posted a fiction recommendation:
Quick mod message - please don't put spoilers above a cut, in this comm! For some people "Not a death fic" is a spoiler! And "Spoiler warnings!" is also a spoiler cos it tells people there are warnings, and therefore something to warn for! There's a long history in Pros of being considerate to people about spoilers and we'd like to carry on with that... If you just put something like the title, or "More information" or something as the lj-cut text, then anyone who's worried can click there to find the spoiler info, and anyone who wants to enjoy the story first can do so. 
Some fans are more sensitive to spoilers than others. Some fans even self-identify as spoilerphobic, while others do not find that their enjoyment of canon is ruined by major or even mild spoilers. The relationship between spoilerphobic and non-spoilerphobic fans is sometimes fraught.
Spoilerphobic fans often request that other fans take precautions to avoid posting spoilery information. Non-spoilerphobes may feel that they are being asked to go to unreasonable lengths in order to protect over-sensitive spoilerphobes. On the other hand, spoilerphobic fans may feel that their requests are entirely reasonable, and a refusal to comply is callous and rude.
Some examples of spoilers are "the Titanic sinks", "Darth Vader is Luke's father", the identity of "the final Cylon" on BSG, and "Snape kills Dumbledore" (the spoiling of which, at the time of the book's release, stirred up much conflict).
- Something Rotten on the Internet by Mary Jean Holmes (2002)
- The Sliding Scale of Story Information, or Labels and Warnings and Spoilers, Oh My!; archive link by cereta (April 1, 2003)
- Why everyone should just read those spoilers anyway; WebCite at The Daily Dot (2015)
- from a rebuttal of a review for You May Deny in Universal Translator #5
- Florahart, PSA regarding whiting out things you want someone to have to highlight to see., accessed June 07, 2009.
- 2010 comment at CI5hq