All HDTV is digital, but not all Digital TV is HDTV. High Definition refers to the resolution of the video (720p, 1080i or 1080p). Digital TV means that the off-air signals are transmitted digitally (usually in ATSC) versus the older analog signals (NTSC). Digital TV broadcasts can be a mixture of standard definition and high definition programs.
1080p is the highest display resolution standard for high-definition television. “1080” refers to the number of lines displayed on a TV screen from top to bottom, while “p” stands for “progressive,” which indicates how the lines are scanned. 1080p is sometimes referred to Full High Definition (HD) to differentiate it from other HDTV resolutions (1080i, 720p, 480p).