A spoiler is an advance revelation of critical plot elements in a book, film, television show, or other form of entertainment media. Spoilers typically include information which readers, viewers, and listeners would be surprised by, if they had a chance to allow the plot to unfold naturally. For some people, this advance information can spoil an experience, hence the slang term “spoiler” to refer to such information.
In order to be considered a spoiler, information must generally not be widely known, and it must be information which would not be unveiled early on in the reading, viewing, or listening experience. For example, saying “Christ is crucified in the Bible” isn't a spoiler, because even people who haven't read the Bible are aware of this critical event. Saying “such-and-such-a-character dies at the end of [about to be released book]” is a spoiler, because it involves the provision of information which is not widely known.
Some people enjoy spoilers and actively seek them out, and generally they have access to a wide range of spoilers, thanks to websites which collect spoilers. Major releases tend to be accompanied by a flurry of spoilers, some genuine and some spurious, and entire sites dedicated to specific television shows, books, films, and other narratives can be found, with users who may argue vociferously about the provenance of various spoilers.
Other people don't like spoilers, and would prefer to absorb a plot as designed by the narrator, rather than having key elements laid out for them beforehand. It has become increasingly difficult to avoid spoilers, especially in the case of major releases, which can be frustrating for people who do not want to be spoiled. Some malicious individuals even go out of their way to expose people to spoilers, as in the case of people who wore shirts reading “Dumbledore Dies” in the days following the release of Harry Potter and the Half-Blood Prince.
Because a spoiler is sometimes critical to a review or discussion of a narrative, responsible reviewers will sometimes include a note to the effect that spoilers are included in their reviews, to warn people ahead of time. On websites which contain a mixture of spoiled and unspoiled material, users may flag spoilers with the words “spoiler warning,” or force readers to highlight spoilers to make them visible, thereby protecting people who do not wish to be exposed to spoilers.