For Zotero for Firefox, click the Zotero icon in the Firefox toolbar to open your Zotero library. This brings up a pane with all of your references, collections, and notes. You can close the Zotero for Firefox pane by clicking on the X icon in the upper right of the Zotero window or by clicking on the toolbar icon again. The Zotero window does not have to be open for you to do “quick saves” of material you want to add to your research collection.
Zotero Standalone can be opened from your operating system's dock or file manager like any other program.
Zotero is, at the most basic level, a citation manager. It is designed to store, manage, and cite bibliographic references, such as books and articles. In Zotero, each of these references constitutes an item.