Here is a portion of the Subscriptions I had in LuckNews before I organized them into folders. As you can see, it looks like a random collection of Subscriptions:
Now go to the File menu and select "New Folder from Selected...". You will be asked for a folder name. I used "Sailing" and hit the Save button. I now have one folder with the given title:
You can also drag and drop Subscriptions into a folder. Simply click on a Subscription and drag it to the folder you want it to be in. You can also drag and drop folders to new locations. Once I do this for a few minutes, I end up with the following:
After having done this, it is now easy to select all the Subscriptions related to a topic to read. For example, if I'm in a hurry I'll read all my sailing related articles, but not the other categories. This is done simply by selecting the Sailing folder. Doing this will in turn select all of the Subscriptions it contains.
Folders can also be used to customize the notification behavior as discussed more in the Customization topic. If you click on a folder and go to the File menu and select "Folder Information" you'll get a preview of what you can customize.