A URI, or Uniform Resource Identifier, is a string of characters used to identify a resource on the internet. It provides a way to access resources such as web pages, images, or files. A URI can be a URL (Uniform Resource Locator), which specifies the location of a resource, or a URN (Uniform Resource Name), which identifies a resource by name in a specific namespace. URIs are essential for web navigation and data exchange, allowing users and applications to locate and interact with resources. They are commonly used in web development, APIs, and data formats like JSON and XML.