Terms you should familiarize yourself with:
Mendeley Desktop : Mendeley Desktop is the downloaded part of the software installed onto your computer.
Mendeley Web : This is the Mendeley website where you can access the web version of your library, edit your profile and search for papers, groups or people.
Sync : The process of synchronizing your Mendeley data across devices.
Web Importer : The browser bookmarklet that lets you quickly import documents from anywhere on the web.
Citation Plugin : A plugin you can install that allows you to create and format your citations and bibliography according to your chosen style.
(from www.mendeley.com)