What's the difference between a style guide and a style sheet?

A style guide is a book that details and prescribes how to handle certain issues that arise in editing. It is not a grammar book (though they will not prescribe incorrect grammar) but is rather designed to ensure consistency. An example of something covered in a style guide would be whether or not to spell out the numbers ten through one hundred. A style sheet, on the other hand, is a document that is specific to an individual book or series and contains details about characters, places, and how words and phrases are handled if contrary to the dictionary / style guide of choice. For example, Merriam-Webster says to spell blood-red as one word (bloodred). I happen to think that looks hideous, as do many of my clients, and so we might include in the style sheet that blood-red should be hyphenated.

Style sheets are generally shorter documents—five to ten pages—whereas style guides can be huge tomes. The Chicago Manual of Style is 1,144 pages long! With that in mind, most publishing houses don't create style manuals (why reinvent the wheel?), and style sheets don't usually contain entries that can be found in other reference books such encylopedias, dictionaries, and style guides, as doing so adds needless bulk to the style sheet and makes it much less useful.

The key purpose of both style sheets and style guides is to ensure consistency, and they are especially important when multiple people work on a project.