what is the difference between 720i and 1080p?

Answer 1

There are 6 types of HDTV: 480i, 480p, 720i, 720p, 1080i, 1080p
interlaced or progressive
480i/p is not HD. 720i is not used by anyone for anything.
720p, 1080i, and 1080p are the only ones worth mentioning then.
Here's the rub -- if you want a digital display (and you do), then there is no such thing as interlaced. Pick your resolution -- 720 or 1080. It'll be progressive. End of story. That's why you get a 1080p display -- because you want to be able to view 1080i/p content without downrezzing it.