A GUID, or Globally Unique Identifier, is a unique reference number used in software development to identify information without the need for a central authority. It is designed to be unique across different systems and databases, ensuring that no two GUIDs are the same, even if generated at different times or locations. GUIDs are typically represented as a 128-bit value, often displayed in a hexadecimal format. They are commonly used in various applications, such as database records, software installations, and network protocols, to ensure that each item can be distinctly identified and accessed.