RSS and XML

RSS is a family of web feed formats used to publish frequently updated digital content, such as blogs, news feeds or podcasts.
Users of RSS content use software programs called "feed readers" or "feed aggregators". The user subscribes to a feed by entering a link of the feed into the reader program. The reader can then check the user's subscribed feeds to see if any of those feeds have new content since the last time it was checked, and, if so, retrieve that content and present it to the user.

The initials "RSS" are variously used to refer to the following standards:
-Really Simple Syndication (RSS 2.0)
-Rich Site Summary (RSS 0.91, RSS 1.0)
-RDF Site Summary (RSS 0.9 and 1.0)
RSS formats are specified in XML (a generic specification for data formats). RSS delivers its information as an XML file called an "RSS feed," "webfeed," "RSS stream," or "RSS channel"

XML
The Extensible Markup Language (XML) is a general-purpose markup language.[1] Its primary purpose is to facilitate the sharing of data across different information systems, particularly via the Internet.[2]
It is a simplified subset of the Standard Generalized Markup Language (SGML), and is designed to be relatively human-legible. By adding semantic constraints, application languages can be implemented in XML. These include XHTML,[3] RSS, MathML, GraphML, Scalable Vector Graphics, MusicXML, and thousands of others. Moreover, XML is sometimes used as the specification language for such application languages.
XML is recommended by the World Wide Web Consortium. It is a fee-free open standard. The W3C recommendation specifies both the lexical grammar, and the requirements for parsing.