A Zotero library is made up of “items”. Items can take several forms: regular items, attachments, and notes.
All items can have tags applied to them and can be linked to related items.
The foundational elements in most Zotero libraries, regular items take the form of reference types—books, journal articles, manuscripts—and have associated bibliographic metadata (Title, Author, Publisher, etc.). Regular items can be created manually by using the New Item drop-down menu or automatically by clicking the Zotero address bar icon to save from a supported website.
You generally always want to work with regular Zotero items, rather than bare attachment files (e.g., PDFs, word processor documents), as regular items have item metadata that is required to interact with most of Zotero's features.
Regular items can only be top-level items. Attachments and notes can be added to regular items as child items.
Attachments are files and web links without full bibliographic metadata. There are four kinds of attachment items:
Web links are essentially bookmarks to websites. When you save a link, Zotero stores only the page title, URL, and access date, and you need to return to the site to view the page content. You can also attach links to the URIs for other programs, such as onenote: or evernote: links.
Snapshots contain the same information as web links, but Zotero also saves a copy of the page as it currently exists so that you can view it later, even if the original webpage has changed or disappeared.
Snapshots can be single files, such as PDFs, or consist of multiple files, as is the case with an HTML page and its associated images.
Linked files are links to files stored outside of the Zotero data directory on your computer.
Imported files are files stored within the Zotero data directory. When you import a file (either using “Store Copy of File” or by dragging in a file), Zotero copies the file to its data directory, leaving the original untouched. After importing a file, you may wish to delete the external copy to avoid confusion.
Each attachment has a single embedded note field.
Attachments can either be child attachments (attached to regular items) or standalone attachments (top-level items not attached to regular items). Attachments cannot have child items attached to them.
As of Zotero 2.0b3, web links and snapshots can only be created as child items (with the temporary exception of PDF snapshots, which can still be created as top-level items to allow for use of the Retrieve PDF Metadata feature).
Notes are pieces of text without full bibliographic metadata.
Notes can either be child notes (attached to regular items) or standalone notes (top-level items not attached to regular items). Notes cannot have child items attached to them.
While the Zotero program will allow you to insert embedded images into notes, they will currently not sync (and make prevent your Zotero library from syncing altogether). Improved support for embedding images in notes is planned.