Use iTunes to copy contacts between Microsoft Outlook and your iPhone, iPad, or iPod Touch. Just like your music and movies, iTunes will let you specify whether or not to sync all of your contacts, or only some groups of contacts.
Once you have specified how you want your contacts synced, if you later make changes in Outlook, or on the iPhone, such as deleting a name or changing a phone number, iTunes will automatically check to see which is the most recent version, then update the older version for you. If iTunes detects substantial changes to your contact database in your computer or on the iPhone, it will ask you before syncing them. This can be a lifesaver if you’ve accidentally deleted half of your contacts.
7 Easy Steps:
1. Connect the iPhone to your computer using the USB cable. Wait for iTunes to open and sync with the iPhone. If you’ve disabled the automatic sync option, then open iTunes manually.
2. Look for your iPhone listed in the Devices section of the left side iTunes. Click it.
3. Click the Info tab at the top of iTunes. 
4. Click the check box beside “Synchronize contacts from.” A list of possible programs is displayed. Click Outlook. Select which groups of contacts you want copied to the iPhone, or select “all.”
5. Click the Apply button at the bottom of iTunes.
6. A dialog box will open warning you that a program is trying to access Outlook and will ask if you want to allow it. This is, of course, iTunes. Specify that the program can have access for ten minutes, then click the Yes button. 
7. Click the Sync button in iTunes. Wait for iTunes to complete its sync, then click the Eject button beside the iPhone.
Sources:
Apple Support: Syncing Contacts and Calendars
