Lists are the ReadCube equivalent of Folders. 

You create a list by clicking the plus symbol at the bottom of your left hand menu:

And then you can just drag and drop papers into that folder!

To create a nested list (folder-within-a-folder), you can click the gear icon beside the folder you'd like to move into another list, and nest it in another list: 

Or, you can also drag a list inside another list:

 To create more layers of organization, we recommend using Tags