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
grammar issues
spelling issues
formatting issues
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.*) (And 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.)
How are Style Guides Used?
A style guide is a very, very good thing for editors. It will keep them from wasting time reworking documents to fit their preferences. (And) A style guide is a very, very good thing for companies and publications. It keeps all their works consistent, which makes their overall offering feel more professional. People might not consciously notice it, but they'll feel like something is wrong if things aren't consistent from page to page. A style guide is also a pretty good thing for writers, because it saves them from spending time waffling about which way to do things (and from fighting about it on e-mail lists). It can also save writers from offending unreasonable editors who have strong preferences that they otherwise fail to communicate. [Note: I would never write very, very in a document, but it's the way I talk, so it's in the transcript.]
So, the bottom line is when you start a project with a new company or publication always try to get the style guide, and if you hire writers make sure you have a style guide to give them.