"Sync" is short for synchronize. When you sync a device, such as a cell phone, PDA, or iPod, you synchronize it with data on your computer. This is typically done by connecting the device to your computer via a USB or wireless Bluetooth connection. For example, you might sync the address book stored on your computer with your cell phone to update the contacts. If you have an iPod, you may connect it to your computer to sync songs, videos, and other data using Apple iTunes.
When you sync a device with your computer, it typically updates both device and the computer with the most recent information. This is also referred to as "merging" the data. For example, if you have added a phone number to your phone since the last time you synced it with your computer, that number will be added to your computer's address book. Similarly, any numbers entered into the computer's address book since the last sync will be added to the phone. Most syncing programs also remove entries that have been deleted on either the device or the computer since the last sync.
Since many devices can be synced with a single computer, the computer is often referred to as the "hub" for syncing portable electronics. For example, you might be able to sync an iPod, Blackberry, and PDA using the same address book on your computer. However, you may need to use a different syncing program for each device, since most use a proprietary software utility to sync with the computer. Common syncing programs include iTunes, The Missing Sync, Palm Desktop, and iSync.