One of many sites explaining it:
http://www.pcworld.com/article/id,128570/article.html
Yes you need to know what you are buying as you may find that you have bought a TV that won't produce what you expect. I think over the past few years lots of people have been caught out by this and bought LCD or Plasma that don't have the correct resolution ect to give you what you want to produce HD pictures.
Look at, "what are 1080i and 720p" take note of the resolution for each and see if your TV comes near to these. yours will more than likely be able to produce 720p and not 1080i.
The best one will be 1080p but this format will be on the newer and more expensive models.
You will find that a 720p produces 70fps and 1080i 30fps so the latter will be better for films and 720p will be used for sports and anything fast moving, so the response time of your TV needs to be low as well.