JMRI uses XML for a number of purposes: to hold decoder definitions, for its persistance system for configuration and panel information, and to create parts of the website from other files.
XML is a text format, and you can edit it with any convenient text editor (though you might have trouble with word processors that insist on reformatting the content).
Still, using an editor that's "XML aware" can make things easier. Those editors can check the syntax as you work, give you useful hints, and provide more powerful editing tools.
This page lists XML editors that have been mentioned on the JMRI users discussion group.
For more information on XML itself, please see our introductory page.
Several users have reported problems with XML Notepad 2007 writing files that can no longer be read by JMRI. This appears to be a problem with its UDF output format.