Radio button	Form input tag used to select one option from among several choices.
RDF	Resource Description Framework is a foundation for processing metadata; it provides interoperability between applications that exchange machine-understandable information on the Web.
Refresh	A meta tag that re-directs the user to another URL after a number of seconds. This is useful when a page has moved to another location.
Resolution	The number of pixels displayed by a monitor, printer or scanner. Resolution is measured in dots per inch (DPI); the more dots per inch, the higher the resolution.
Robot	A Web agent that visits sites to extract data from the site. It automatically traverses the Web's hypertext structure by retrieving a document, and recursively retrieving all documents that are referenced. Most commonly, data from robots is used to build the catalogs for search engines. Web robots are sometimes referred to as Web Wanderers, Web Crawlers, or Spiders. These names are a bit misleading as they give the impression the software itself moves between sites like a virus; this not the case, a robot simply "visits" sites by requesting documents from them.