From the NCTA website
What kind of equipment do I need to receive HDTV?
You will need to purchase a digital television capable of displaying a high-definition signal, which uses either the 1080i or 720p format. Most high-definition digital televisions will display these formats. In some digital television sets, the digital decoding and tuning functions are built into the TV. Other instances require a digital monitor, capable of displaying HDTV, plus a tuner in a separate set-top box. When receiving HDTV from cable, an HD set-top box may be purchased at a retail outlet or leased from the cable operator, depending on the local cable system.