A lot of computer programs nowadays store their data files in XML format. XML, or Extensible Markup Language, uses a hierarchical tree struture to store data in plain text files. It's very similar to HTML, but can be used for anything you like, not just for storing web pages.
In many cases, XML-based data files are hard to recognise as such, because the program which creates the file doesn't give it an XML extension. For example, I use a writing app on my iPad called Index Card, which syncs its data files to my Dropbox account. If I look in my Dropbox, the data files have a .indexcard extension but, if I view one of them with standard Windows Notepad, there's a telltale <xml> tag at the top of the file, which tells me that this is an XML file.
Although you can view XML files with a simple text editor such as NotePad or WordPad, a proper dedicated XML editor can make your life easier, as you can navigate cleanly through the structure of the data. Once excellent free XML editor is called XML Notepad, and it comes from Microsoft.
You can get it from http://www.microsoft.com/en-us/download/details.aspx?id=7973 and it's only a 2 MB download. And although it dates from 2007, it works perfectly on my Windows 8 installation.