1080i is interlaced, which means the picture is made up of 2 scans, 1 consisting of the odd number of lines in the image (lines 1,3,5,7 etc) and the second made up of the even number of lines. (2,4,6,8 etc)

This creates a complete image but in 2 scans.

Progressive displays all the lines in the image at once.

1080P is the best resolution currently on the market