In addition to items and file attachments, you can also store notes in your Zotero library: child notes, which belong to a specific item, and standalone notes. Notes are synced along with item metadata, searchable, and support HTML formatting (Zotero uses the TinyMCE HTML editor for note editing).
To create a child note, select an item in the center column. Then either click the “New Note” button at the top of the center column () and select “Add Child Note”, or go to the “Notes” tab in the right-hand column and click the “Add” button. You can also right-click an item (ctrl-click on OS X) and select “Add Note”.
A note will be created as an attachment to the item (it will also show up under the “Notes” tab), and a note editor will appear in the right-hand column. You can create a dedicated window for the editor by clicking the “Edit in a separate window” button at the bottom of the editor. Text in notes is saved as you type.
Standalone notes work the same as child notes, but are not directly related to any item in your library, and will appear alongside any other items in your library. To create a standalone note, click the “New Note” button and select “New Standalone Note”.
With Zotero for Firefox, you can also create notes directly from the webpage you're visiting in Firefox. Highlight the text you want to copy into a note, right-click (ctrl-click on OS X), select “Zotero” in the pop-up menu, and select “Create Zotero Item and Note from Selection”. This will create a new “Web Page” item in your library for the visited webpage, with a note containing the selected text.
While it is not possible to search notes while using the note editor, their contents is searchable by using the regular Zotero search. To search the text of a single note, there is a workaround for Zotero for Firefox: right-click the note, and select “Generate Report from Selected Item…”. The note report, which will open in the current Firefox tab, can then be searched using the standard Firefox text search tools.