Hopkirk, I will try to give you a simple answer.
Televisions display the image in horizontal LINES across the screen. We dont see the lines but they are constantly being refreshed so we see an ever chaning image.
Obviously the more lines on a TV the better the quality. The first TVs many years ago only had 32 lines, then 405 lines, then 625 lines and so on. Current analogue TV is 625 lines.
When HD came out they upped the number of lines, either 720 or 1080. Obviosuly 1080 lines is the best of the two.
But there are two types of 1080, one with a letter i at the end (1080i), one with a letter p at the end (1080p). 1080p is the best and is "proper HD".
So if you buy a 720 lines TV it will SHOW HD but not at the best quality. This will be marked as "HD ready".
If you buy a "Full HD" TV it will be 1080p lines, and will show HD at the best quality (as long as the image coming in is HD like SkyHD or a BluRay disk).