Most of the time, you'll want your iPad to sync automatically, either via USB or WiFi. But sometimes you don't have time for a full sync, or don't want syncing to take over your iPad just yet. If your device is set to automatically sync via iTunes, a simple keyboard command can override the sync settings for a single session.
On a PC, hold down the Shift and Control keys when plugging in your iOS device to override your automatic sync. Mac users can accomplish the same thing with the Command and Option keys.
If you have multiple devices, such as an iPhone and an iPad, it's simple to sync one but not the other automatically. To do so, plug in your device, and in the Summary tab, check or uncheck the box labeled Open iTunes when this [device] is connected. Do that for each of your devices, leaving the box checked for the ones you want to sync automatically, and unchecking it for the others.
On the other hand, if you want to turn off auto syncing for all of your devices, in iTunes, go to Preferences > Devices and check the box marked Prevent iPods, iPhones, and iPads from syncing automatically.