If you own an iPhone, or are considering buying one, you should download this 9 MB PDF file from Apple It's the official iPhone user guide that is packed with a lot of helpful information.

This page on Apple's website is a great place to get help on your using your iPhone (including Activation, Frequently Answered Questions, etc.) That page also contains links to full specifications and system requirements and host of other helpful information.

This Apple Discussion forum is dedicated to the iPhone and is worth knowing about

This iPhone Service FAQ Page is worth reviewing as it covers a range of important issues. In particular, it addresses how to get a loaner iPHone if yours is in for service (battery replacement or other warranty or non-warranty issues). Well worth the read!