EXACTLY how does a Wireless Card Work? How will I be able to access the internet from anywhere with this card?

I want to know how it works. Example: If im sitting outside at one of the tables of "The Corner Bakery" in any given city, with my laptop, how is it that i am able to access the internet? Am i "hacking" in to anybody else's signal? I dont want to do that. Can someone, courteously, explain to me the ins and the outs of such a devise. And what's the best kind of wirelss card to purchase? How is it billed? By minute usage or what? HELP!!! : ) Thank you ALL in advance. Amen!