In English, as with many languages, there are certain writing rules may not necessarily be uniform across all platforms. These variations are called writing styles. A style guide is a book, manual or other type of reference that provides standard writing rules. Eventually, writers who regularly refer to a particular writing style guide may only need to refer to it in rare circumstances.
In academic writing, most instructors will require one of three different writing styles. These include the AMA Style, devised by the American Medical Association; the APA Style, devised by the American Psychological Association; and the Chicago Style. There is a style guide available for each writing style. In publishing, The Chicago Manual of Style is often considered the standard.
The AMA style guide and the APA style guide generally concern themselves with how to cite sources. This includes both parenthetical (in text) citations and the bibliography style. In some cases, the citations may be nearly identical. However, it is the subtle nuances that professors and other instructors often look for in order to make sure the writer is in compliance with the proper style.
While these two style guides often are most concerned with the references, they may offer some guidance for other rules as well. However, The Chicago Manual of Style goes into much greater detail concerning things like punctuation use, use of numericals and even spelling. While it would be hard to make sure every single punctuation and numerical is used correctly, it is the writer's job to make it as close as possible.
Another common type of writing style, which is also has a very extensive style guide, is AP Style. Invented and endorsed by The Associated Press, this style is often the one used by newspapers around the world. For example, saying there are three apples and those apples weigh 3 pounds are both written according to the AP style guide even though one uses three spelled out and the other uses a numerical. The AP style guide directs all numbers less than ten must be spelled out unless they are used in certain ways, such as with measurements, including weights.
While writers may not like style guides because they can seem burdensome and demand more detail than most will ever notice, they do play a vital role in providing uniformity across a wide spectrum of writers. Without that uniformity, it may seem as if all types of writing is simply thrown on the page in a haphazard fashion, with many different writers choosing whatever feels comfortable at the time. This will lead to credibility issues as readers begin to expect a certain style but may not be provided it in all cases.
In most cases, style guides should be available in the reference section of a bookstore or library. Some may also be available as reference sheets provided by an instructor. Some academic books, while perhaps not providing a complete reference in the style, will provide enough information in order to allow students to write a paper in accordance with the instructor's expectations.