http://support.apple.com/kb/ht1766
iTunes will back up the following information:
With iPhone OS 1.1 and later
Safari bookmarks, cookies, history, and currently open pages
Map bookmarks, recent searches, and the current location displayed in Maps
Application settings, preferences, and data
Address Book and Address Book favorites
Calendar accounts
Wallpapers
Notes
Call history
Mail accounts
YouTube bookmarks
SMS messages
Saved suggestion corrections (these are saved automatically as you reject suggested corrections)
Camera roll (photos and screen shots taken by the iPhone)
Voicemail token (this is not the Voicemail password, but is used for validation when connecting. This is only restored to a phone with the same phone number on the SIM card.)
Web clips
Network settings (saved wifi spots, VPN settings, network preferences)
Paired Bluetooth devices (which can only be used if restored to the same phone that did the backup)
Keychain (this includes email account passwords, Wi-Fi passwords, and passwords you enter into websites and some other applications. The keychain can only be restored from backup to the same iPhone or iPod touch. If you are restoring to a new device, you will need to fill in these passwords again.)
With iPhone 2.0 and later
Managed Configurations/Profiles
List of External Sync Sources (Mobile Me, Exchange ActiveSync)
Microsoft Exchange account configurations
Nike + iPod saved workouts and settings
App Store Application data (except the Application itself, its tmp and Caches folder).
With iPhone 3.0 and later
Videos in camera roll
Per app preferences allowing use of location services
Offline web application cache/database
Voice memos
Autofill for web pages
Trusted hosts that have certificates that cannot be verified
Web sites approved to get the location of the device
In-app purchases
New with iPhone 3.1
Videos in the camera roll that are 2 GB or larger are not backed up.