Mendeley is  a reference manager for MS Word, OpenOffice and LibreOffice and  an academic social network. After downloading the desktop application, you MUST create a (free) online account before you can start using the application. Although it's not strictly required, you can upload your references and grant other users access to them. This is of course fine if you want to collaborate with others. You can also create research groups for your particular research field. Although you don't upload anything yourself, you can still search and download references from the mutual online pool.
I searched for a famous book which was first published in 1943 and which has been reprinted innumerable times since then. To my surprise the book itself wasn't there, but instead I found scholarly journal articles ABOUT the book.
Citation plugins for MS Word, OpenOffice (?) and LibreOffice need to be installed/activated extra. If you use other and better word processors such as Nisus Writer Pro, Mellel or Scrivener, you're completely out of luck: there seems to be no easy way to enter citations and automatically create bibliographies in anything else than in Microsoft Word and LibreOffice. You can do it manually by copy and paste, of course, but who would want to do that? Zotero doesn't have this glaring omission. For this reason I give Mendeley only 1 star. For citations and bibliographies I recommend Bookends. It's extremely versatile and has the best support I have ever experienced by any software company.