References can be organised into ‘Collections’, another name for folders, and sub-collections. Collections are displayed in the left pane of the Zotero library.
You can organise your references by putting them into folders called Collections, rather like organising your files in 'My Computer' or your emails. You can also build multiple levels of collections in a hierarchical structure.
To add references to a collection:
Select one or more references from My Library.
Drag and drop them in the collection you want. This will create a copy of the reference(s) in the collection while the original reference will remain in My Library.
If you remove a reference from a collection you will still have the original in My Library, but if you remove a reference from My Library, it will also remove it from any collections to which it has been added.