Sync is on your computer iTunes.

Read here:

Sync with iTunes

Syncing with iTunes copies information from your computer to iPhone, and vice versa. You can sync by connecting iPhone to your computer, or you can set up iTunes to sync wirelessly with Wi-Fi. You can set iTunes to sync music, videos, photos, and more. For help syncing iPhone, open iTunes on your computer, choose Help > iTunes Help, then select Add items to iPod, iPhone, or iPad. iTunes is available at www.itunes.com/download.

Set up wireless syncing. Connect iPhone to your computer. In iTunes on your computer, select your iPhone, click Summary, then select Sync with this iPhone over Wi-Fi.

If Wi-Fi syncing is turned on, iPhone syncs when it’s connected to a power source, both iPhone and your computer are on and connected to the same wireless network, and iTunes is open on your computer.

Ways to sync with iTunes on your computer

Connect iPhone to your computer, select it in iTunes, then set options in the different panes.

• If iPhone doesn’t appear in iTunes, make sure you’re using the latest version of iTunes, check that the included cable is correctly connected, then try restarting your computer.
• In the Summary pane, you can set iTunes to automatically sync iPhone when it’s attached to your computer. To temporarily prevent syncing when you attach the device, hold down Command and Option (Mac) or Shift and Control (PC) until you see iPhone appear in the iTunes window.
• In the Summary pane, select “Encrypt iPhone backup” if you want to encrypt the information stored on your computer when iTunes makes a backup. Encrypted backups are indicated by a lock icon , and a password is required to restore the backup. If you don’t select this option, other passwords (such as those for mail accounts) aren’t included in the backup and you’ll have to reenter them if you use the backup to restore iPhone.
• In the Music pane, you can sync your entire music library or just selected playlists, artists, albums, and genres.Note: If you turn on iCloud Music Library, you can’t use iTunes to sync music to iPhone.
• In the Photos pane, you can sync photos and videos from a supported app or folder on your computer.
• If you have iCloud contacts or iCloud calendars turned on, you can’t use iTunes to sync contacts or calendars from your computer.
• If you turn on iCloud Photo Library, you can’t use iTunes to sync photos and videos to iPhone.

What are you trying to sync to? What are you trying to sync?

If you're syncing to iCloud, you chose what you want to sync in Settings>iCloud. See this article for more information:

http://www.apple.com/icloud/setup/