Item - a reference or a source of information. Zotero uses "items" to represent references for books, articles, etc.
Library - a master folder of organized references. The default "My Library" is personal and cannot be shared.
Collection - a child folder (sub-folder) found in your Library. You can also have sub-collections within a collection.
Group - a Library used for sharing references with others. There are 3 different types of Group Libraries:
- Public, Open Membership - Anyone can view your group online and join the group instantly.
- Public, Closed Membership - Anyone can view your group online, but members must apply or be invited.
- Private Membership - Only members can view your group online and must be invited to join.
For more information, visit the Zotero documentation topic on groups.