HDTV, standing for high-definition television, is a new means of television broadcasting and the machines that take advantage of it. HDTV broadcasts video digitally (in contrast to the common analog formats PAL, NTSC, and SECAM) and of higher 720 pixels or 1080 pixels resolution.
Source: http://www.freemake.com/blog/what-is-hdtv-everything-you-need-to-know-about-it-faq/