XML (Extensible Markup Language) is an Internet-friendly format for data and documents, invented by the World Wide Web Consortium (W3C). "Markup" denotes a way of expressing the structure of a document within the document itself. XML has its roots in a markup language called SGML (Standard Generalized Markup Language), which is used in publishing and shares this heritage with HTML. XML was created to do for machine-readable documents on the Web what HTML did for human-readable documents that is, provide a commonly agreed-upon syntax, so that processing the underlying format becomes a commodity and documents are made accessible to all users.
Unlike HTML, though, XML comes with very little predefined. HTML developers are accustomed to both the notion of using angle brackets ( < > ) for denoting elements (that is, syntax), and also the set of element names themselves (such as head, body, etc.). XML shares only the former feature (i.e., the notion of using angle brackets for denoting elements). Unlike HTML, XML has no predefined elements, but is merely a set of rules that lets you write other languages like HTML. (To clarify XML's relationship with SGML: XML is an SGML subset. In contrast, HTML is an SGML application. RSS uses XML to express its operations and thus is an XML application.)
Because XML defines so little, it is easy for everyone to agree to use the XML syntax and then build applications on top of it. It's like agreeing to use a particular alphabet and set of punctuation symbols, but not saying which language to use. However, if you're coming to XML from an HTML background (and have an interest in extending RSS), then you may need to prepare yourself for the shock of having to choose what to call your tags!
Knowing that XML's roots lie with SGML should help you understand some of XML's features and design decisions. Note that although SGML is essentially a document-centric technology, XML's functionality also extends to data-centric applications, including RSS. Commonly, data-centric applications do not need all the flexibility and expressiveness that XML provides and limit themselves to employing only a subset of XML's functionality.