The following definitions will hopefully clear up the any questions you may have about ATSC, NTSC, and QAM.
NTSC (National Television System Committee) - Old analog broadcast format orginally developed in 1941 and modified in 1953 for color. TVs that receive TV signals over-the-air built with NTSC tuners will require a digital-to-analog converter box to continue viewing television signals after the digital transition. Read more about NTSC at Wikipedia
ATSC (Advanced Television Systems Committee) - Digital television format defined by Advanced Television Systems Committee which will be replacing NTSC (analog) broadcasts on June 12, 2009. The switch from analog to ATSC is also known as the Digital Transition. TVs with a built-in ATSC tuner will not require a digital converter box, and may even be capable of receiving and displaying High Definition programming. Read more about ATSC at Wikipedia.
QAM (Quadrature Amplitude Modulation) Tuner - For use with subscribed television services to a cable company. TVs or set top boxes with a built-in QAM tuner are capable of receiving high definition cable programming of local channels without a cable set top box. Read more about QAM at Wikipedia.
Please use this thread to discuss any questions, comments, or anything related to the ATSC format, NTSC format, or QAM Tuners. Please post if you would like to add any additional information that I may have missed.