It's actually not that complicated. The number represents the total lines of resolution and the letter stands for either progressive or interlaced.
With interlaced you only ever actually have half of the lines of resolution showing at any one time....every other line to be exact, and then it flickers over to the lines it wasn't showing before, but it does it so fast that the human eye cannot detect it.
With progressive scan you have all of the lines of resolution on your screen at all times.
To sum it up, if you have a 1080p tv you are really getting 1080 lines of picture all the time, but with 1080i you are getting 540 lines at a time, but because both sets of 540 lines are interlacing so fast it looks like 1080.
Now one might think then that 720p is actually more hi-def than 1080i, and in some cases it's true, but not always. 1080i looks better than 720p for most things, but if you were to watch a video of a fast moving object, the object itself would look more crisp on a 720p. Interlaced video has slight blurring on the edges of fast moving objects.
Of course all of this means nothing if your tv is smaller than 40 inches because the human eye won't know the difference.
