Depends on the state. Here in FL (like many states) it's a sales/use tax:
If you paid sales tax in the state it was purchased in, you pay the difference (if any) of Florida's 6% state sales tax. (I.E. if I paid 4% to Utah, I'd have to also pay 2% to Florida)
The use tax is a major screwing, it's time specific. Something like after six months you can get hit for the 6% on top of the other tax you paid in the purchase state.
Go to your state's revenue site and search "mail order" or "sales/use" or "out-of-state purchases".
Here's your state's law:
http://www.mass.gov/?pageID=dortermi..._use&csid=Ador
You didn't really think they'd let you off the hook did you?