A Uniform Resource Identifier (URI) is a character string that serves as a unique reference for resources on the Internet. These resources may be abstract or physical. Possible resources are text, video and audio files, as well as web services such as email, programs for data transfer or network adapters.
The concept of the Uniform Resource Identifier forms the basis for different protocols on the Internet. URIs represent the syntax on which protocols such as HTTP or FTP must be oriented in order to work faultlessly. However, URIs are not a specific grammar but rather a meta-concept that allows different types of uniform identifiers to be formulated. The individual URIs, which must conform to different criteria depending on the system, are generated by a schema that is closer to a specific grammar than the meta-concept. This means that URI is an abstract concept that creates a uniform identifier only in the different application areas using the respective schema. The supreme premise is always the uniqueness of this identifier, so that a given object, for example a network adapter, can be exactly referenced.
There are three basic types of uniform identifiers: names, places, and individuals. Names can be used to identify the content of a resource in order to differentiate it from a variety of resources. If a location is specified in the identifier, a resource is identified by means of a network address. In general, URIs can also refer to abstract entities, for example, to identify a person using a mobile phone number or email address.
Each URI starts with a scheme that defines how URIs are formulated within this scheme. With the authority element, a URI can be associated with a privileged access to the resource in question. If user information, host, and an optional port are specified here, these are authority subcomponents that govern access to the resource. The path element specifies the path of a resource. Absolute and relative paths are possible. The Query element can identify a resource using attribute-value pairs without accessing hierarchically sorted data, as is the case with the authority element. Fragment is an element that allows indirect identification using other resources without having to access them directly.
In practice, URIs are always used, but in many cases it is not obvious that they are a URI. The URL (Uniform Resource Locator) should be mentioned as a subtype. If you surf the Internet, you can only navigate between different web addresses using URLs. A URL is an application example of a URI. The scheme is designed to accurately determine a location in a network. URI and URL are often used synonymously, incorrectly so. If a URL is specified as a unique reference, it is called a Permalink in technical terms.
URLs, the subtype of URIs, are particularly important for search engine optimization. During the planning stage of a Website, the URL structure should be taken into account to give users and search engine references to the contents of the website. The basic structure should not be changed once the page has been crawled. The same applies to each URL within the structure. Because the search engines can rate a newly formulated URL Duplicate Content, if a different, but content-matching URL exists. When choosing URLs, not only relevant keywords that describe the page content should be used, but also terms that speak for themselves. This means that such terms are used which describe the content of the pages in simple words and do not confuse with specialized vocabulary or long character string with parameters. The latter is often the case with dynamically generated websites. However, there are solutions such as the mod_rewrite module of Apache Servers that can rewrite a dynamic URL into a static one.