A URI (Uniform Resource Identifier) is a standardized string of characters used to uniquely identify a resource on the internet or within any computing system. The resource in question can be a web page, an image, a document, a service, or virtually any named entity - the URI simply provides a consistent, unambiguous way to refer to it.
URI vs. URL: Understanding the Difference
The most important distinction to understand is the relationship between a URI and a URL (Uniform Resource Locator). A URI is the broader, more general concept: it encompasses any identifier for a resource. A URL is a specific type of URI that not only identifies a resource but also describes how to locate it by specifying its access mechanism and network address. In other words, every URL is a URI, but not every URI is a URL.
To make this concrete, consider the web address https://example.com/page. This is both a URI and a URL, because it identifies a resource and tells a browser exactly where and how to retrieve it. By contrast, a URI like urn:isbn:978-3-16-148410-0 - a Uniform Resource Name identifying a book by its ISBN - is a URI but not a URL, since it names the resource without specifying any location or retrieval method.
Structure of a URI
A URI is composed of several optional and required components defined by RFC 3986, the internet standard that governs URI syntax. At minimum, a URI contains a scheme - the prefix that indicates what type of identifier it is, such as https, mailto, ftp, or urn. Beyond the scheme, a URI may include an authority (typically a domain name), a path, a query string, and a fragment identifier. The exact combination of these parts depends on the scheme being used.
Why URIs Matter for the Web and SEO
For web developers and SEO professionals, URIs are foundational. Every resource that a browser requests - a page, a stylesheet, a script, an API endpoint - is addressed by a URI. Search engines index and rank content based on the URIs that identify it, which means URI structure directly influences how content is discovered, crawled, and understood. Clean, descriptive URI paths contribute to both usability and search visibility.
Because URIs are plain text strings transmitted over the web, certain characters must be encoded before they can be safely included. This process, known as URL encoding (also called percent-encoding), replaces unsafe characters with a percent sign followed by their hexadecimal code - for example, a space becomes %20. This encoding applies to URIs broadly, not just URLs.
Understanding the URI as the overarching concept helps clarify the terminology used across web standards, browser documentation, and HTTP specifications, where the terms URI and URL are sometimes used interchangeably in informal contexts, even though they carry distinct technical meanings.