XML stands for eXtensible Markup Language and is a popular way of representing data for storage and transfer across a network. XML is a very flexible format and is used to represent many types of data. The current specification of HTML, which powers most of the web, is an implementation of XML.
The version of XML that we will concern ourselves with is RSS, which stands for Really Simple Syndication. RSS is used to define a collection of time-ordered pieces of digestible content; these RSS feeds can then be used to aggregate content from a number of different sources. RSS is typically used as a distribution mechanism for news articles and podcasts.