Your Mendeley account gives you access to your Mendeley library on the desktop version and online. Synchronising the two means you’ll see the same references in either version.
To sync, click the Sync button on the toolbar or use the File menu and select Synchronise.
When you first go into Mendeley Desktop, it will automatically synchronise with the web version and add any new references you have added. The Edit Settings button at the top of the page next to All Documents allows you to choose to synchronise file attachments in selected folders.
If you use the web version whilst the desktop version is open, you can tell it to synchronise by clicking on the Sync icon. If you make changes in your desktop library, it is important to remember to Sync your library so that any changes are fully saved.
When you first open Mendeley Desktop you will be asked to confirm that Mendeley may back up your library to the Cloud. You can change your preferences at any time by clicking on the Edit Settings button at the top of your library.