What is the difference between sync-ing my iPhone and backing-up my iPhone?
Backup means copying your personal data from your phone to your computer. That includes application settings, text messages, voicemails, etc.
Sync means copying new apps, music, video, or books from iTunes to your phone, or from your phone to iTunes.
You can think of it this way: backup saves a copy of stuff you've created, and sync copies media you have downloaded via the iTunes Store.
Backup copies everything, but Sync only copies what you choose through iTunes. By default, it copies everything, but you can fine tune it all you want. You might want to stop movies from syncing to your phone if you never use it to watch movies, for example.
Unless you've changed the settings, when you connect your iPhone to your computer, iTunes always performs a backup first, then a sync.
When you connect a new iPhone to your computer, iTunes will offer to reconfigure it using the latest backup it has. So if you connect your old iPhone, let it backup/sync, then connect your new iPhone, it should transfer everything so that your new phone is just like your old one, but faster!
Syncing your iOS device to iTunes
Syncing your iOS device with iTunes allows you to copy information(more detailed list below) to your iOS device from you computer and vice versa. Here is an Apple support article on backing up your iOS device which provides more details.
http://support.apple.com/kb/ht1386
As per the article you can sync the following
- Applications
- Audio content—music, podcasts, audiobooks, and iTunes U content
- Bookmarks
- Books
- Contacts
- Calendars
- Movies and TV shows
- Photos
- Notes
- Documents (File Sharing apps only)
- Ringtones
Backing up and Restoring your iOS device
When you connect your iOS device to your computer it will perform a backup before it will continue to sync automatically. A backup saves all the information on your iOS device to your computer so that if you get a new iOS device or in your case an iPhone 5 your computer will ask you if you would like to perform a restore which will copy all of your settings from your old device to your new device. And here is another Apple support article about syncing your iOS device.
http://support.apple.com/kb/ht1414
1 - Sync
Copies new data from your iPhone (or other device) to iTunes and removes deleted data. Also does the same backwards from iTunes to your device.
So to answer your question, yes things might change here and there, but only to make the devices "match" each other in the end.
Note that SYNC also does BACKUP as one of its steps!
You should sync if you downloaded new media, apps, or updates.
2 - Backup
Simply creates a "snapshot" of your device. Can be used to "roll back" your device to this state if it acts wacky one day.
Of course you should do this only when your device is feeling awesome and shiny (or before you give it to your 2-year old niece to mess up all the apps in it).
3 - Restore
Restores your device to the state when it was feeling awesome and shiny.
