Collections help you better organize your references. Collections are similar to file folders on your computer; however, in Zotero a reference can be in more than one collection at a time. For example, an article about Virginia Woolf could be filed in your "Women and Writing" collection, your "British Authors" collection, and your "Woolf" collection without having to make multiple copies. You can see your collection on the left side of your Zotero Standalone, Web, or Plugin.
You will find information Zotero has collected about your references on the right side of your library. Bibliographic information, like author, journal title, page number, etc. can be found in the 'Info' tab. This includes the title, author, and publication year. By clicking on any field you can change the information or add to it. The only fields you cannot edit under 'Info' are 'Date Added' and 'Modified'.
If there is a PDF with an article citation, Zotero will often save the PDF automatically to the citation. If it does not save automatically, check your Zotero Preferences. To attach items in your Zotero library, you must change your Zotero settings.
You can also attach files and PDF's manually to citations.