What's unique about the app, is that it allows you to store all of your contacts, tasks (calendar), notes, and to-do lists divided into tree-structured categories. In addition, each item (be it a note, task, contact ,etc) may belong to several categories (tags).

So, this is a combination of folders (tree) and tags. Note, that if you create subfolders (subcategories) - then by opening a folder you can see all the items belonging to it, and the contents of all its subfolders. So, by opening root folder (at the top), you can see all items of given type (e.g. notes, if you're in "notes" tab).