Unlocking your iPhone is done so that you may use any SIM Card from any service provider on a GSM network, truly making it a world phone. iPhones typically are "locked" to a specific network by the service provider you buy your iPhone from. To unlock your iPhone, the iPhone Dev-Team have a program called "Yellowsn0w" To find info on this and instructions on how to unlock go here: http://yellowsn0w.com/
Jailbreaking your iPhone frees it so that 3rd party apps, games, themes etc... can be used on your iPhone. This will allow personal customization and use outside of the restrictions Apple has put in place. Thus, an iPhone "Jailbreak". Instructions on how to Jailbreak are posted in the Blog Archive under "How to Jailbreak your iPhone 3G 2.2" posted on Tuesday, January 27, 2009. If you have Firmware 2.2.1 then go to the post in the Blog Archive under "**UPDATE** 2.2.1 Firmware Jailbreak" posted on Friday, January 30, 2009.
Hope that clears up the confusion!
Stay iTuned for more to come!