You can organise your references into collections and subcollections. A single reference can be in more than one collection. If you delete a collection or remove references from a collection, the references are not deleted from your Zotero library.
To create a collection: click on the 
button on the left, and enter the collection name. Once a collection is open, every reference you create or download is added to that collection. You can also add references to a collection by dragging and dropping them onto the collection in the left pane.
To remove a reference from a collection, right-click it and select Remove Item from Collection. To delete a collection, right-click it and select Delete Collection.
To create a subcollection, create a collection and drag and drop it to the parent collection, or right-click on a collection icon and select New Subcollection.
If you want to know which collections a reference has been placed in, you can select the reference and click on the Ctrl key (the Alt key on a Mac), and the collections will be highlighted.
By default, items added to a subcollection do not appear in the parent collection. To change this, select the parent collection, go to the View menu and select "Show Items from Subcollections".
To create an automated collection (a collection linked to a search query): use the advanced search function and enter your search terms, then click on the Save search button.