How does the apple ipad 3g work?

if I get the 3g that is 15 dollars and comes with 250 mb. what does the 250 mb do? like what do i have to do to get it used up? Like when I download stuff does it take up mb? Also if I do use up the mb then can i turn it off and use my wifi at home? I was only going to get the 3g if I was like away from any wifi but if im like at home where wifi is available can i turn off the 3g network and conserve the megabyte and also just use my wifi?