A hard reset will delete everything on your device. All your data will be deleted, and formats, preferences and other settings are restored to their factory default settings. Don't perform a hard reset unless you've backed up your data, or you're willing to lose everything.
Want to back up and restore your data?
Be sure to perform regular HotSync operations to back up items. That way, if you need to reset your device in a way that deletes all data, you'll have a backup that you can use to restore your data
HotSync note: Some third-party applications change the way your device backs up data. Other third-party applications do not back up data. If you have third-party software on your device, please contact the developer to learn what components are backed up during a HotSync operation, and if HotSync conduits are altered. Any data not backed up will be deleted.