Yes, there is a big difference between 720p and 480p or 480i. These numbers refer to the vertical resolution of the image. The little p or i means progressive or interlaced.
480i is a standard, non-hidef television signal.
480p comes from a progressive scan DVD player.
720p signals are HDTV signals, currently broadcast only by ABC and ESPN, I believe. Some XBOX games are also 720p.
1080i signals are HDTV signals broadcast by just about everyone besides ABC.
I would strongly recommend that you take a few days or weeks to learn a little more about the current crop of TVs before jumping in and buying a TV tomorrow.