Universal selector
In CSS, it selects any element within the document.
u
URI
URI is an abbreviation that stands for Uniform Resource Identifier. It is a codification of addresses to internet objects. For example, a URL and a URN are both URIs. URIs are described in RFC 1630.
URN
URN stands for Uniform Resource Name. It is the address of a persistent object on the Internet. The name of an object would not change, while the location, or URL, might. RFC 1737 and RFC 2141
Usability
Usability is the process of making a website easy for customers to use. It encompases the heuristics of the site as well as the methods that people use to manipulate the site. A website is considered usable if the customers coming to that site can find what they need and accomplish their goals.
User Agent
Every customer need to be identified by the server or internet apps through a user agent, which is any program used for accessing a Web site, and that identifies some parameters about the client such as language, OS, browser version....
Amazon is an online shop holding an affiliates program being used by many websites, so we can find links<br />to shop books in Amazon within many different websites.