iTunes, in addition to syncing calendars, apps and email accounts, can also take care of your Contacts organisation. To sync your contacts, iTunes will require you to connect specifically with one computer. Syncing is also bi-directional - any changes made to a contact on your device will show up on the computer the next time you connect the two, and vice versa.
Use iTunes to sync contacts with Outlook, Address Book or Google
Sync with the Mac
On the Mac, your device can sync contacts from OS X's Address Book, an online Yahoo Address Book or your Google contacts. The first time you connect your phone to your Mac, iTunes will ask which contacts you want to import. You can choose to import every contact in OS X's Address Book or specify only selected groups.