UGC : An acronym for User Generated Content.

uptime : The percentage of time a Web site is up and running, not including scheduled maintenance periods.

URL: An acronym for Uniform Resource Locator.

User-Agent HTTP header : Information describing whether a visitor to a Web site is a person or a search engine robot.

user-agent sniffing : A process that identifies spiders coming to a Web site, enabling the Webmaster to keep out bad spiders.