Sync is a two way service that allows you to synchonize your phone’s built-in address book and calendar with your Google account wirelessly i.e. without been connected to your computer. Your calendar and contacts are always up-to-date, no matter where you are or what you’re doing. Also, since your data is automatically backed up to your Google Account, it’s securely stored even if you lose your phone.
For devices that support the open SyncML protocol, Google Sync will allow for two-way contacts synchronization. If you’re a BlackBerry user, a version of Google Sync is already available.
Once you have setup Sync on your phone it will automatically update your phone’s contacts with your contacts from your Gmail account as well as download your Google calendar events to your device.