Synchronizing compares the content of your music library to the portable device and downloads the necessary copies of files to that device. 

Windows Media Player:

To synchronize content to a portable device, Windows Media Player must establish a “relationship” with that device.  This happens when you connect your device to the computer for the first time.  At this time Windows Media Player starts the Device Setup Wizard.  You must decide whether you want the content in your library to be synchronized to your device automatically or manually.  If you choose automatically, then the device will be synchronized every time it is hooked up to the computer.  If you choose manually, then every time you connect your device to the computer, synchronization will not begin until you start the process. 

RealPlayer

In RealPlayer, you may synchronize in two different ways.  The first is to synchronize manually.  To do this, select Synchronize Manually from the sidebar in the Burn/Transfer page (your device must be attached to the computer).  You may also set up RealPlayer to synchronize automatically any device that is attached to the computer.  To do this, select Edit Options… from the sidebar in the Burn/Transfer page.  Click the Synchronization tab and select Enable automatic synchronization…