A style manual determines how spelling, punctuation, and other elements of your work should be handled.
For instance, some guides say the serial comma — the one before “and” when listing items in a series — should be omitted unless the lack of a comma causes confusion. (She sang to her parents, Harold and God.) Other guides say to always use the serial comma.
Which is correct, “Web site” or “website”? According to the CD-ROM version of Merriam-Webster’s 11th Collegiate Dictionary, “Web site” is correct. But. The online version now says “Web site” or “website.” The 16th edition of The Chicago Manual of Style uses “website.” What’s an editor supposed to do? Pick one and use it consistently.
A manuscript will usually require, in addition to a general stylebook, its own style sheet — a listing of proper names, author’s preferred spelling of certain words, and so forth.
The standard style guide for books published in the United States is The Chicago Manual of Style. The standard for newspapers is The Associated Press Stylebook and Briefing on Media Law.
And then there’s the Publication Manual of the American Psychological Association, the MLA Style Manual and Guide to Scholarly Publishing, the AMA Manual of Style, The Bluebook: A Uniform System of Citation, the United States Government Printing Office Style Manual, et cetera, et cetera, ad infinitum.