Online Sales "Use Tax"?

I don't think this has been addressed here; I checked the search. But forgive me if it has.

I just received in the mail a letter from my Accountant informing me of the California "Use Tax" for out-of-state (and therefore 'tax-free') online (or regular) purchases.

Am I living in a closet? I've never heard of this. Have you?

I thought purchases out-of-state and online were just 'tax-free'. I guess I'm just a bit naive (maybe a little stupid) but maybe I should have thought about it. However, if I buy something in Pennsylvania and they don't tax me and they ship to me in L.A., why on Earth would I pay California sales tax on that item? Yeah, it's taken revenue away from the state when I'd otherwise buy in California...oh, wait, I get it.

So now the Accountant is suggesting that we declare our online purchases on our tax return.

We don't do a lot of online purchasing out-of-state or much online purchasing, for that matter. Yet. But I know a lot of my BOTL/SOTL do.

So, will you declare your online out-of-state/out-of-country "use tax"?