Style guides represent a physical or digital document that describes styles, patterns, practices, and principles that build a specific brand/company and teach others how to use them. In other words, it documents the visual identity of a brand or company and how it's used in multiple places.

A style guide may include:

• Design principles
• Best practices
• Brand styles (like colors, logo, or social icons)
• Use cases
• Writing style and tone
• Iconography
• Imagery

Style guides ensure that designs are consistent across a product and brand. For reference, take a look at Foursquare or Duolingo style guides.