From Wikipedia:
A style guide or style manual is a set of standards for the writing and design of documents, either for general use or for a specific publication, organization or field. The implementation of a style guide provides uniformity in style and formatting of a document.
What are the most popular such manuals? Which are considered "standards" for writers to follow? Are they used by specific professions, industries, etc.?
To add on to TML's answer:
Journalists generally use the AP Stylebook.
When writing for the web, the Yahoo! Style Guide is often used.
The style manual or guide depends on the audience. Journalists will use the AP Stylebook. Academic writers have a few more options depending on their field. Most English or Literature focused writing will use MLA and social science writing will likely use APA. For other kinds of writing, especially for an academic journal, check what style they prefer. They may have their own style (ACS) or use something like Chicago.
My experience indicates that the "big three" are: The MLA Style Manual, Strunk and White, and The Chicago Manual of Style.
MLA (Modern Language Association) is the style guide used in the academic world in essays and term papers. It's generally used in English, History, Literature, and similar classes.
APA (American Psychological Association) is the style guide used in the academic world for Psychology and Science type courses.
CMOS (Chicaco Manual of Style) is the style guide used by 90% of publishing houses. Most houses also have their own style guide that takes its information from the CMOS and shows only the relevant information.
Style guides such as these, Chicago, the AP stylebook, and the MLA handbook explain the structure that documents should have, and provide detailed instructions about such things as capitalisation of titles. They are much less widely read than usage guides such as Strunk & White's Elements of Style or Fowler's, which are references for writing effectively, but which don't care about the purpose of the writing, or whether it will ever be published.
Not a style guide, but for adding nuance to this discussion I'd like to suggest the talk "Linguistics, Style and Writing in the 21st Century" by Steven Pinker, held at the Royal Institution. Summary:
Does writing well matter in an age of instant communication? Drawing on the latest research in linguistics and cognitive science, Steven Pinker replaces the recycled dogma of style guides with reason and evidence. [...] Steven argues that style still matters: in communicating effectively, in enhancing the spread of ideas, in earning a reader’s trust and, not least, in adding beauty to the world. Steven Pinker is an experimental psychologist and one of the world’s foremost writers on language, mind, and human nature. He is Professor in the Department of Psychology at Harvard University and conducts research on language and cognition but also writes for publications such as the New York Times, Time, and is the author of many books, including The Language Instinct and How the Mind Works.
The Apple and the Microsoft style manuals (do not have the exact names with me to provide) are valuable for as style guides for software documentation and any writing about computers. They were formerly available easily on the websites, but have become harder to find. Maybe someone has archived these? Search around in MSDN and you'll find the current Microsoft UI documentation guidelines.
They were THE definitive guides for whether "double-click" has a hyphen (it does), what to call an interface thingy that you click to select one of several options (MS calls it an "option button", Apply calls it a "radio button" and that sort of thing).
