RSS or Atom
RSS (nowadays) stands for Really Simple Syndication. It is implemented by means of an XML file that can be read by an RSS reader application. RSS is typically used to provide links to updates on websites that change frequently (such as blogs). Wikipedia has a good entry on the rather tangled history of RSS. The specification is available at the Berkman Center for Internet & Society at Harvard Law School. Atom is a competing specification which is championed by Google and is, unlike RSS, an official Internet Engineering Task Force standard, see the RFC.