About OPML
As defined by Wikipedia, OPML is:
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 OPML specification defines an outline as a hierarchical, ordered list of arbitrary elements. The specification is fairly open which makes it suitable for many types of list data.
That's just an elaborate way of saying that an OPML is a plain text file containing a list of the sites to which you subscribe to via RSS.
Creating your OPML
OPML files are generated by your feed reader. You should be able to find export instructions in the documentation of the application or website. Here's a few links to instructions to some of the more popular readers:
