While their contents may seem mysterious to those outside the world of print publishing, many writers in English have heard of such style guides as the Chicago Manual of Style (CMOS), the New Oxford Style Manual, and the Publication Manual of the American Psychological Association (APA).
Even though some of their content might seem more relevant to those who design publications themselves, many of the rules and principles set out in these guides have an impact on English-language writing globally. For example, in March, 2017 it was revealed that the upcoming CMOS revision would switch from Internet to internet and from e-mail to email!
While the scope of each style guide is different, for editors and savvy writers the heart of each "style" consists of a set of rules, which generally includes a citation format (or formats, in the case of CMOS) as well as standards for the layout and titles of tables and figures. Rules for how to communicate numbers in the text, how to punctuate quotations, whether to place a comma before "and", and even how many spaces should follow each period are questions to which the answers may differ for each style guide.
Therefore, one of the most important things writers can do for their editors -- and for themselves -- is to know what style guide they mean to follow.