Style guide
A style guide is a written reference that fixes the repeatable choices in a body of work: spelling, capitalisation, punctuation, terminology, formatting and, in visual guides, the mechanics of using type, colour and imagery. Its purpose is to remove per-instance debate, so the same question is not answered twice.
Style guide is not Brand guidelines.
Brand guidelines govern identity and its permitted expression: what the brand is, what the logo may do, which colours and which tone belong to it. A style guide governs the mechanics of executing that, the house spellings, capitalisation and formatting rules that would otherwise be re-argued on every piece.
In practice
The value of a style guide is not elegance, it is the elimination of repeated small decisions. Does the product name take a capital in the middle of a sentence. Is it email or e-mail, five or 5, per cent or percent. Each question takes seconds and none of them matters much alone, but they recur on every piece of work and every person answers them slightly differently. Written down once, they stop being decisions at all.
The form comes from publishing, where a house style keeps a magazine written by forty people sounding like one publication. The well-known guides, whichever your industry uses, are essentially long lists of settled arguments. Brands borrow the form and extend it: alongside the language rules sit the mechanical visual rules, minimum logo sizes, clear space, which colour combinations are permitted, how a photograph may be cropped. The unifying property is that every entry answers a question that was going to come up again.
The common mistake is writing the guide as an act of completeness, covering every case at maximum length. Long guides do not get read, and rules that are not read are not followed, which produces exactly the inconsistency the document was meant to prevent. A useful guide is short at the start, ordered by frequency rather than by category, and grown by accretion: when the same question is asked twice, add the answer and nothing else.
Keep it next to the work rather than in a drive nobody opens. The entries with the highest return describe a recurring judgement rather than a fact: how to refer to customers, how formal to be in an apology, whether contractions are allowed, what to do when a sentence needs a number and a symbol. Those are the questions people otherwise resolve by instinct, and instinct varies by author and by mood. Fixed values such as hex codes and font weights belong in the kit; the judgements belong in the guide.
It is worth being honest about the ceiling. A style guide can only govern the surface. It will keep spelling, spacing and terminology consistent across a hundred pieces, and it will do nothing about whether any of them says something worth reading. That is the job of positioning and voice, and mistaking a tidy document for a coherent brand is how organisations end up perfectly consistent and completely forgettable.