I'm sure this has been asked before, but I guess I have 3 questions here about jail breaking:
1. What functional value does it have or what can I do(in general) that I wouldn't be able to do otherwise?
You can add apps from sources other than the app store
You can add 'short cuts' and change the way the phone works. Press the volume up button to compose a new text, respond to text messages without closing out of an app, start apps without unlocking the phone, stuff like that.
2. How difficult is it for a relative iPhone rookie to do?
If you're decent with computers (can install a new program, can figure out simple problems by searching the internet, etc.) you should be able to figure it out pretty easily.
3. What is the danger or down side of doing it?
Some apps 'interfere' with each other, and can cause the phone to be buggy. If an app does slow down the phone, you can just delete it, and the problem should be fixed.
Don't be worried about the warranty being voided. If you don't like your JB'ed phone, you can just restore it to the stock OS, and all evidence of your JB is gone. Apple will then replace your phone (if necessary).