A JavaScript Style Guide is a set of rules and guidelines for writing JavaScript code. It ensures consistency and improves readability by standardizing syntax, formatting, and best practices across a codebase. Common elements include naming conventions, indentation rules, best practices for variable and function declarations, and guidelines on commenting. It helps developers write code that is easy to understand and maintain, especially in team environments. Popular examples include the Airbnb Style Guide and Google’s JavaScript Style Guide. Adherence to a style guide is crucial for collaborative projects and maintaining code quality.