Brand guidelines
A documented set of rules governing how a brand may be expressed: logo use and clear space, colour values and where each is permitted, typography and hierarchy, imagery treatment, voice principles, and the things explicitly not allowed. The purpose is to make correct execution reproducible by people who were not in the original decisions.
Brand guidelines is not Style guide.
Brand guidelines govern the whole expression of a brand: marks, colour, type, imagery and voice. A style guide governs execution mechanics: spelling, capitalisation, punctuation, terminology, formatting and, in visual guides, how type and colour are applied. Guidelines decide that the voice is direct and unhedged; the style guide decides whether it is 'email' or 'e-mail'.
In practice
Brand guidelines exist because brands are executed by people who were not in the room when the decisions were made, and often by the person who made them, eighteen months later, at eleven at night. Their job is to convert a set of judgements into a set of instructions that can be followed without judgement. If a designer, a contractor and a founder can each produce work from the guidelines and the three results sit together without argument, the document is doing its job.
The usual failure is a document that describes rather than governs. Pages of mood, a rationale for the logo, a paragraph on what the colours evoke, and then nothing that says the minimum clear space, which colour a button may be, or whether the wordmark may sit on a photograph. Length is not the measure. A four-page document with values, roles and prohibitions is more useful than an eighty-page deck that only explains itself, because the questions that get asked in practice are narrow and operational.
Good guidelines are specific about three things people otherwise improvise. Values: exact colour codes for every space the brand is reproduced in, named typefaces with permitted weights, fixed logo variants with minimum sizes. Permissions: which combinations are approved, which backgrounds each logo version may sit on, which pairings meet contrast requirements. Prohibitions: the stretched logo, the unapproved tint, the drop shadow, shown as examples of what wrong looks like. The prohibitions section is usually the most consulted page and the one most often left out.
The other half is verbal, and it is the half most guidelines are missing. Colour and type are governed carefully while the writing is left to instinct, which is why brands whose visuals hold together can still read like three different people. Guidelines should carry the voice principles, the vocabulary that belongs to the brand, the words it refuses, and worked examples of the same message written correctly and incorrectly.
In practice, guidelines only work at the point of use. A PDF in a shared folder loses to whatever is already open in another tab, and every colour picked by eye is a variant nobody will ever clean up because nobody knows it exists. This is why guidelines increasingly live inside the tools that produce the work, and why a system that scores a brand profile field by field, and flags which parts have gone stale, is worth more than a document that is correct on the day it is published and quietly wrong afterwards.