They represent the number of horizontal lines of pixels on your screen. The more lines you can squeeze onto a screen, the more realistic the image will appear.
Standard definition for US TV's is 480 lines (576 for European TV's). All standard definition TV's, no matter how large, will have the same number of lines but they get spaced further apart or the pixels are bigger meaning the image can look worse with a larger screen.
To fix this problem, they've upped the resolution (number of horizontal lines of pixels) to 720 and even 1,080. Now even on very large TV's the image is incredibly clear.
Now the difference between "i" and "p" has to do with how the TV draws the lines on the TV. It takes a lot of power to draw everything on the TV and to make it easier, TV manufactures use an "i"nterlaced feature. The TV only draws half the lines, alternating, at one time for each frame. On the next frame, the TV draws the other lines. This is done faster than the eye can see BUT....TV's that can render "p"rogressive scan can draw all lines at once per frame. The difference is a smoother animation especially in high speed scenes in movies and games.
I hope that helps.
The rEVOLution is not being televised
