Jailbreaking an iPhone allows you to install third party applications that are not directly available from Apple's App Store.
The process is reversable.
Why Jailbreak:
http://sleepers.net/why-jailbreak/Guides:
http://thebigboss.org/2008/11/22/22-jailbreak-posted/This process is not free from danger or potentially damaging your warranty.
If this is a concern, use your iPhone as Apple intended, you can enable Disk Usage from within iTunes. This only allows access to certain 'disk' areas, but it might suit your needs.
