HDTV has a greater number of picture elements (pixels) than SDTV (Standard Definition Television). SDTV is made up of 525 scan lines of pixels going from the top of the picture to the bottom. HDTV has upto 1080 scan lines. The various resolutions are now set forth in visible scanlines. In SDTV only 480 of the scan lines are visible so it is referred to as 480i. HDTV comes in one of three resolutions 720p, 1080i or 1080p. The "i"s and "p"s refer to Interlace or Progressive. Interlace means that the picture is made in two passes with the TV first showing the odd numbered lines, then the even numbered lines. Each pass is called a field and and a SDTV picture "frame" is made up of two fields. Progressive means that the picture "frame" is made in one pass. The best resolution is 1080p, but you will only receive that resolution from HD-DVD or Blu-Ray Discs. Over the air broadcasts will use either 720p or 1080i.
HDTV also has a wider aspect ratio. The aspect ratio is the ratio of the pictures width to its height. SDTV has an aspect ratio of 4:3 that is four units wide to 3 units high. HDTV has an aspect ratio of 16:9.
Finally, HDTV is from a digital source, while SDTV is from an analog source. The FCC has mandated that all over the air Television signals will be digital no later than February, 2009. While television stations have the option of sending out their digital signals at 480i, most have also converted to HDTV resolutions.
The importance of that last point is that come February 2009, all the SDTVs in the United States will become obsolete and incapable of receiving over the air television transmissions. Unless you purchase a convertor box.
Hope this helps.