If you installed the Zotero Connector for Firefox, Safari or Chrome, you are able to save citations automatically from within your browser to your Zotero library.
Multiple Citations
On some web pages that list information about multiple items (e.g. a list of database search results), Zotero will show a folder icon. Clicking this folder icon will open a window where you can select the items that you want to save to your library.
Sometimes Zotero can't recognize the type of resource you are viewing in your web browser. Even if Zotero can't automatically capture citation info from a certain web pages, you can still add them to your library.
Saving Screenshots
Taking a screen shot saves a copy of the webpage to your computer, which means if the page is removed later or if you're offline, you'll still be able to view your copy.
Compatibility
Zotero provides an official list of compatible sites, but keep in mind that many other sites not included on this list work with Zotero as well.
If references aren't showing up, you may need to sync your library (especially if you use both Zotero Standalone and and Zotero Online). To do this, click on the circular green arrow in the upper right corner of the program.
You can also add references manually. You can do so by clicking the "New Item" button, which is a green circle with a white plus sign inside:
From there, select the appropriate item type. If you hover over "More," that will give you many other options. After that, you can add the other metadata (like title and author) in the right column.
Zotero can add items via their ISBN number, Digital Object Identifier (DOI), and PubMed ID.