Items in Zotero libraries can be organized with collections and tags.

Collections allow hierarchical organization of items into groups and subgroups. The same item can belong to multiple collections and subcollections in your library at the same time. Collections are useful for filing items in meaningful groups (e.g., items for a particular project, from a specific source, on a specific topic, or for a particular course). You can import items directly to a specific collection or add them to collections after they are already in your library.

Tags (often called “keywords” in other contexts) allow for detailed characterization of an item. You can tag items based on their topics, methods, status, ratings, or even based on your own workflow (e.g., “to-read”). Items can have as many tags as you like, and you can filter your library (or a specific collection) to show items having a specific set of one or more tags.

Tags are portable, but collections are not. Copying items between Zotero libraries (My Library and group libraries) will transfer their tags, but not their collection placements. Both organizational methods have unique advantages and features. Experiment with both to see what works best for your own workflow.

Full Instructions: Collections and Tags