About HD

HDTV means High Definition TV. This is as opposed to Standard Definition or SDTV.

Picture sizes

Larger numbers of pixels to represent the image is the essence of HD. Better quality images through the higher resolution.
Sizes include 1080, 720, and 480.

1080 is of interest in the UK, 720 and 480 are only used as broadcast formats in the US.

The numbers refer to the number of active lines, i.e. lines which actually contain picture information.The actual number of lines for 1080 is 1125, but the format is never referred to as 1125. Note that the reverse situation exists for SDTV where the PAL format is referred to as 625 which includes all the vertical interval, the number of active lines containing picture information being 576 (The NTSC format is known as 525 and has 486 picture lines).
HD is a 16x9 square pixel format.

This means that if there are 1080 lines, then there will be 1080 x 16 / 9 = 1920 pixels in each line.

Therefore HD has a resolution of 1920 x 1080