I was going to suggest XML also. The structure that comes to mind is along the lines of creating an XML file that describes the categories, and includes a directory name for each category. Then in each category directory, a file for each thing, possibly with an index XML file that lists all the things. Depending on your comfort level with XML and your comfort level with plain text, you might find it easier to do the same thing with plain text. You can pick different storage methods for each level (part) of the data structure, depending on what that data needs. (For example, you didn't specify how much data is associated with each category, so if it's just a name, there might be no need for a description of each category, just a set of directories, like /usr/data/Category1, /usr/data/Category2, etc.