Wikipedia :
High-definition television (or
HDTV) is video that has
resolution substantially higher than that of traditional television systems (standard-definition TV, or
SDTV, or SD). HDTV has one or two
million pixels per frame, roughly five times that of SD. Early HDTV broadcasting used analog techniques, but today HDTV is
digitally broadcast using
video compression.
From Matthew Torres, former About.com Guide : High definition is the top-level resolution offered within the digital television category. Digital cable comes in three formats - standard, enhanced, and high definition. Standard has a resolution of 480i, enhanced is 480p, and high definition is 720p and 1080i. Therefore, HD is digital, but not all digital is HD.
Steve Conner, Science editor : HDTV stands for high-definition television and it should mean better quality pictures and surround sound. The proponents of HDTV say that viewers will be able to see individual blades of grass on a football pitch and the details of faces at the back of a crowded stadium - as well as the beads of sweat on David Beckham's neck.