It usually cost money and depending on your provider can be pricey. To tether on to your ipad from your iphone though I believe you have to have an adapter as well so the usb could plug in to your ipad port.
Oh and I forgot to explain it to you when you tether your iphone to your computer or any device your phone becomes the modem. You can then connect to the internet using your wireless data plan for your phone if you dont have an internet connection around. So it basically allows you to use your computer from any where you want as long as you have a phone signal.
I don't understand technology at all but I pay O2 ( uk ) £6 bolt on a month And on my iPad I go into the settings and find personal hotspots. I have used it but there are so many places with free wifi I don't tend to. Hope that helps.