In most online resources, Zotero automatically detects when you are looking at information for a book, article, or other source and inserts a "save reference" icon right into the address bar. The address bar icon will look like a book, document, newspaper, etc. To save, simply click the icon. If you are looking at a list of references, you may see a folder icon. This icon will prompt you to select multiple items from the list to save.
Zotero senses a list of items, so the 'folder' icon is displayed. You will be prompted to select items to import to Zotero.
Zotero senses a single citation, so the 'article' icon is displayed.
Zotero sees a book citation, so the 'book' icon is displayed.
If Zotero does not automatically detect (provide a "capture icon" for a list, book, or article) to import a reference into your library you can use the export feature provided by the database.