Products vs Services and In vs Out of State taxes as related

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Hello E-commerce gurus, I have noticed a lot of sites make a difference between the "home" state where a company operates. Namely, if a transaction is invoked by a "resident" user (at least by his or her signup account) the local state taxes are levied. On the other hand if the "user" is: "out of state" then no taxes are levied. Can anyone explain how I may view the statute or law that states this situation? Or is their a definitive website article somewhere that outlines and details the "internet" tax laws? The question I am most interested in: what are the tax responsibilities and/or differences between a website that is selling a product and a website that is selling a "service". Thanx.