As far as I understand it there are a few options that some carriers are offering:
1. USB tethering, you connect your phone to another device (laptop) via a usb cable and the phone serves as the 3G modem for that device.
2. Wireless tethering, same as above, only wireless, still serves only one device.
3. Wireless mobile hotspot. Basically wireless tethering for multiple devices where the phone acts as a 3G modem and router.
The only one that I can see paying for is #3 since it allows you to overuse your data plan, i.e. share it with others not already paying for the service. But if I'm already paying AT&T for data why should it cost more to use it in a different way?