Brand guidelines are a set of tools and rules on how to use your branding elements. The guidelines are used by designers, writers, web masters and anyone else using your brand’s elements (like your logo) to create marketing materials.
The guidelines are usually presented in a pdf format that covers how your brand works and how the different elements of your brand work together to form your brand identity.
Brand guidelines are used as a tool to keep your brand consistent. Whether your business is large or small, (even you're a solopreneurs and freelancers), if you have a logo, brand guidelines are critical to helping your brand to work effectively. Without these guidelines, your brand’s message can change at any point just because a logo was used improperly or because an outdated logo was used.
Your Brand Guide may look very different from someone else's. Depending on the nature of your business, it may include sections on photography or use of stock images, styles for industry icons or when to use a wordmark instead of a full logo.