A style guide is simply a set of rules as to how your business produces it’s communication materials (including website content, letters, emails, marketing documents and promotional articles.)
By having a style guide, you can ensure everything you present to customers and potential customers is consistent and supports your brand. For instance, if someone reads a formal letter from you then visits your casual website, they will notice the difference and probably feel uncomfortable with it.
Style guides can be in bullet point on one page or they can be comprehensive manuals – it depends on the needs and size of the business. In fact, I have written both types for a single client as they used them for different purposes.
If you want to create a style guide, you can always start with the key points and slowly build it up as you gather further information to include.
A style sheet is a summary of a style guide that lists common words and how they are to be presented. For instance, does your business write Internet or internet? Or is Aussie acceptable or must it always be Australian?
P.S. I wrote a longer comparision betwen style guide and style sheet last Novemeber.