What does “720p” mean?
The number 720 is the number of lines of vertical display resolution, while the letter p stands for progressive scan or non-interlaced.
“Progressive” simply means that every time the image is refreshed (between 24 and 60 times each second, depending on the format), the whole image is presented. “Interlaced” on the other hand, only displays every other row of pixels per refresh, but by alternating odd and even rows, appears to the human eye, to display the whole picture.
Due to the way they work, 720p and 1080p are better than their “i” counterparts (720i and 1080i respectively) for display on LCD “flatscreen” monitors, but are less effective when displayed on CRT screens.