Image resolution is the noticeable detail in an image. Given that pixels are the smallest point of data in television pictures, it would seem that comparing pixel quantity is the best way to evaluate relative res. Most non-HD TV sets have a maximum of 480 vertical interlaced lines, otherwise known as 480i. Therefore the picture consists of 480 lines of information in every frame. There are 30 frames per second. In each frame, there are two fields. Standard televisions show two fields in each frame. A single field displays the 240 positive scan lines and one field shows the 240 negative scan lines in every frame. This is termed interlacing and accounts for the "flicker" you notice on standard television sets. This same rule applies to HD TVs with 1080 lines.
In 1080i for every 1/60th of a second fifty percent of the 1080 lines of resolution are updated while on 1080p all 1080 lines are updated. The "p" in 1080p, is short for "progressive scan." Both have 1080 lines, but only half are updated every 1/60th of a second at 1080i. You'll typically see this difference (if you're able to notice it at all) in color brightness along with rapid action shots. Most people may detect a small enhancement in the image between the two. The bottom line is that 1080 progressive scan is the maximum refresh cycle you will get.
The more economical 720p format when compared with 1080p is barely noticable on mid size televisions. Chances are you'll only notice the visible difference on the larger sized televisions.