A subset of a brand is the brand identity (also called corporate identity or identity system). The key word is identity. Just like with people, checking an ID proves you’re you and not somebody else. The tangible elements you can see when a company communicates with you make up its identity design:
Logo, colors, fonts, icons, letterhead, business cards, envelopes, websites, packaging, uniforms, office aesthetics, promotional swag, social media, email blasts, signage, messaging.
A brand style guide is a document that records this identity. It keeps everyone on the right track, using the right fonts, colors, and more.