How does this whole thing work?

My sister is applying for a British passport, as her father was born here.  What will that do to her tax arrangements?  

Is she just planning on getting it and not moving to the UK? 

If so then it shouldn't affect anything, the US is the only country, that I am aware of, that taxes its citizens no matter where they earn their money. 

She's planning on applying for it so that she can move here for a few years and work, so the tax implications are especially important for me to be able to detail them to her (she's 18).  I need to know how much if any the US will be taxing her in addition to the already exorbitant rates we pay in this country.

Foreign earned income to a US citizen in the UK is excluded up to $87,600. Which means that if she lives & works in the UK her income up to that level will not be subject to US income tax, however she will still need to file a return each year regardless.

From the IRS:
http://www.irs.gov/businesses/small/international/article/0,,id=97130,00.html

That is exactly the answer I was looking for, thank you so much!  Have a lovely day, all.  : )

That is exactly the answer I was looking for, thank you so much!  Have a lovely day, all.  : )

You're most welcome