Why HD is better

SDTV

480i - 720x480 interlaced (NTSC)
480p - 720x480 progressive (NTSC)
576i - 720x576 interlaced (PAL)
576p - 704x576 progressive (PAL)

HDTV

720p - 1280x720 progressive
1080i - 1920x1080 interlaced
1080p - 1920x1080 progressive

A picture to explain the numbers:

HD vs. SD

Check out the fine details that are lost with lower resolutions:

HDTV comparison

All we need now is a permanent vacation!