Yes - if you try to sync, then it will tell you that it is a new device and everything will be erased. When you sync, you are syncing with the data in the actual iTunes Library. You select which Apps, Music, Movies, TV, Photos, Books, etc. that you want to sync to your device in the Device Profile tabs that will display in iTunes when you hook up your device. Whatever you have selected will sync to your device.
If you have your Contacts, Notes, Calendars, etc. already checked to sync with iCloud (not backup, but the items that are tuned on under Settings>iCloud on both your device and your computer), then those will not get replaced.
If everything on your device is purchased from the iTunes Store, you can download it all to your computer by going to the appropriate tab in the iTunes Store, click on Purchased under the Quick Links section to the right, and then download anything that has a cloud next to it.
Here is a support article on syncing:
iTunes 12 for Windows: Sync iPod, iPhone, or iPad
Cheers,
GB