1920x1080 Widescreen Wallpapres Desktop:

1080P means 1920x1080 Progressive Scan. Which means all 1920x1080 pixels are updated in a single pass(Odd & Even Lines).

1080i means Interlace Scan. Which means odd and even lines(horizontal lines) of the 1920x1080 are updated alternately. The effective scan rate is 30HZ.

1080P and 1080I have the same 1920x1080 pixels. The 1080P has better quality as the entire screen is updated at 60HZ. The 1080i is updated at half the rate.  1920x1080 defines the pixel matrix of the screen (i.e. 1920 vertical pixels and 1080 horizontal pixels).