The iPhone’s iOS does a good job at providing users with plenty of reset options. Usually, a reset is an “all or nothing” kind of thing. With the iPhone though, you can reset single things like the keyboard dictionary or just the settings you’ve changed over the course of owning the device. Here’s how you can access each of these options.
Reset all settings
Choose General > Reset and tap Reset All Settings.
All your preferences and settings are reset. Data such as your contacts list and media such as your songs and videos are not deleted.
Erase All Content and Settings
Choose General > Reset and tap Erase All Content and Settings.
All your data and media are deleted. You must sync iPhone with your computer to restore contacts, songs, videos, and other data and media.
Reset the keyboard dictionary
Choose General > Reset and tap Reset Keyboard Dictionary.
You add words to the keyboard dictionary by rejecting words iPhone suggests as you type. Tap a suggested word to reject it and add your word to the keyboard dictionary. Resetting the keyboard dictionary erases all words you’ve added.
Reset Network Settings
Choose General > Reset and tap Reset Network Settings.
Source: iPhone Users GuideLast modified March 22, 2012