720i, 720p, 1080i and 1080p ???

Very confuse of the termology.

First of all, we need to standard the naming convention.

i = Interlaced scan
p = Progressive scan.

Interlaced scan means in one 'pass' every other horizontal line on the screen is drawn, and on the next pass, the remaining are drawn. This has less picture quality than progressive scan, which draws all the lines in each pass. 720, 1080, are just the number of lines. 1080p is the top end right now.

720i does not exist.
No 1080p is being broadcast right now.
It's either 720p or 1080i.

720p = 1280 x 720 x 50** = 46 million pixels of details
1080i = 1920 x 1080 x 25* = 52 million pixels of details
1080p = 1920 x 1080 x 50** = 104 million pixels of details

*1/25 second is the time taken for interlaced piture to scan its top+bottom fields
**1/50 second is the time taken for progressive scan picture to scan both fields

For more details about progression and interlace:
http://hometheater.about.com/od/beforeyoubuy/a/progressivescan.htm