HDTV is 1,080 lines of dots from top to bottom of a TV. DVD is 480 lines of dots from the top to bottom. Digital TV, is 480, standard TV is 280 (or something like that).
There is a concept of "i" or "p". "i" being Interlaced(480i), "p" being Progressive (480p). When your TV draws the picture, "i" says to build it one line at a time, but skip every other line and build the whole view in two passes. "P" is the opposite or non-interlaced to computer monitor junkies.
480i is nice
480p is better (except where noted)
540p is usually doubled and called HD, if you double 540, you get 1080i
720 is another variation out there, but not used *very* often.
1080i HDTV
Depending on the type of set you have... the line doubler in it... the size of the power supply... the chipset of the progressive scan DVD, you may not see a difference between a non-progressive (480i) and a progressive scan (480p) DVD. Best places to look, dark pictures or heavily shadowed. Non-P will show a shadow, P will show more detail inside of the shadow. Some would say... BFD
FOX HD, or "Fox widescreen" is really.... 480p. In otherwords, not "HDTV".
CBS has the most content, followed by NBC. Fox and ABC are catching up. Content overall has doubled pretty much every year for the past 5 years.
HD is available from DirecTV, DishNet and even using a standard antenna. The local channels broadcast either non-HD, or both Non-HD and HD. They are on two different channels.
TV either has an HD reciever, or you need to add one. Comcast (in some areas) my cable provider, has a cable box that is the standard digital cable box, but also receives HD content. In this case, you TV must be able to do HD, not necessarily have a HD Receiver in it.
I can go on for hours if you would like... LOL