It's true that when it comes to grammar there are a lot of hard and fast rules, but it's also true that there are hundreds, maybe thousands, of issues that are dictated by style.
It's true that when it comes to grammar there are a lot of hard and fast rules, but it's also true that there are hundreds, maybe thousands, of issues that are dictated by style. I know it would be so much easier if the rules were just black and white, and I could always just tell you what to do—I like to tell people what to do—but in a lot of cases you're just going to have to decide on your own.
Grammar Rules Aren't Always Black and White
Here's an example: there is an e-mail list for writers that I subscribe to where the people are practically in a flame war right now about whether there should be one or two spaces after the period at the end of the sentence. These people are surprisingly militant about spaces. Honestly, it kind of scares me; but regardless of what you think about the issue, the bottom line is that it's enough of an unresolved point that it's a matter of style. You should just find out what the style is of the people you are writing for and do it that way.
What Is a Style Guide?
So, back to the topic: what's a style guide? A style guide is a document that is typically put together by editors, managers, or producers to define how they want their writers to handle all the unresolved writing and grammar problems that arise (and, believe me, they do arise on an almost hourly basis.) It can include almost anything the creator wants it to, but a style guide typically covers things like
general writing recommendations
A grammar issue might be whether to capitalize the first letter of a full sentence after a colon. (I don't.) A spelling issue might be whether to use the American or British spelling of a word. (I use American spelling.) A formatting issue might be what font to use for a specific section of the document or web page. (If I'm referring to a specific word in the blog, I make it bold.*) Finally, a general writing recommendation might be whether jargon is allowed. (I try not to use jargon, or at least to define it when I do use it.)
Next: How Are Style Guides Used?