Please note that the plan states Tethering. In case you do not understand what tethering means, some cellphones have the ability to be connected to a computer via Serial/USB (Data Cable). You then use the Cellphone as a Wireless modem accessing the internet through your Data Plan on your computer.

PDA/BLACKBERRY PLANS WITH TETHERING
PDA/BlackBerry plans with Tethering may ONLY be used with AT&T-certified RIM BlackBerry devices and PDAs  for the following purposes: (i) Internet browsing; (ii) email; and (iii) intranet access (including access to corporate intranets, email, and individual productivity applications like customer relationship management, sales force, and field service automation). PDA/BlackBerry plans with Tethering may be used to tether such PDA and BlackBerry devices to a Personal Computer. The parties agree that AT&T has the right to impose additional charges if you use more than 5 GB in a month.  Prior to the imposition of any additional charges, AT&T shall provide you with notice and you shall have the right to terminate your service.