Several potential pathways exist for improving your iPhone's function if the device begins to operate sluggishly over the course of your use. One of these is to use iTunes' "Restore" function. As its name suggests, restoring your iPhone resets it to what it was when it left the factory.
Reasons for iPhone Restore
Several potential reasons exist for restoring the iPhone. Most commonly, chronic iPhone problems are the motivator. In some instances, perpetual sluggishness in starting up, loading applications or even delays in typing on the device's keyboard may be to blame. More serious issues may also cause you to want to restore your iPhone, such as if applications frequently crash or if the device turns itself off, even when the battery is fully charged.
Restoring your iPhone resets it to its factory settings, which means any customizations you've set over its lifespan are deleted. These include modifications as simple and generic as setting different ringtones for messages and numbers or the image you set as your iPhone's background, to more custom settings, such as keyboards in different languages and the formatting of time and date on your device. Regardless of complexity, any change you made to the device since you purchased it is reset when you restore.
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Contacts, Multimedia and Other Data
An iPhone restore also wipes your device's flash drive clean. For example, it deletes all the contacts you have in your phone book, as well as logs of your calls and all text messages you've sent. It also deletes the music you have stored in the iPhone's iPod; photos and videos from its Camera Roll; and any applications that didn't come standard on the iPhone, such as Facebook or UrbanSpoon.
How to Backup and Restore
If you need to restore your iPhone, don't stress out about losing all your data -- iTunes automatically prompts you to back up your data before you restore. To get started, connect your device to iTunes, click its name and click the "Restore" button in the "Summary" tab. When iTunes prompts you to back up your device, click "Yes." After it informs you the backup is complete, click "Yes" when it asks you to continue with the restore. Disconnect your device when the restore has completed, then reconnect it and launch iTunes. Right-click the name of your device and click "Restore from Backup" to replace data onto the iPhone.