720P/720I = 1280x720
1080P/1080I = 1920x1080
P = Progressive Scan, which to my understanding takes more bandwidth to transmit, but results in higher quality
I = interlaced which takes less bandwidth (It alternates transmitting odd numbered lines in one frame and even numbered lines in another and your mind blurs it together, to my understanding)
720P = the same quality as 1080I if my understanding is correct