Wikipedia describes OPML as follows: OPML (Outline Processor Markup Language) is an XML format for outlines (often blogrolls). Originally developed by Radio UserLand as a native file format for an outliner application, it has since been adopted for other uses, the most common being to exchange lists of web feeds between web feed aggregators.
The specification of OPML 2 is given at http://dev.opml.org/spec2.html. There is a specific section of this specification entitled Subscription lists that describes how to provide an OPML file where each item describes a newsfeed.
A validator for OPML is available at http://validator.opml.org/.