Hi Def TV Explained on Wikipedia
Quite in depth, the graphics made me realise what the numbers meant (it's actually referring to screen resolution) and all in all - a great source if you wanna know pretty much everything there is to know.
HDTV resolution explained
Found this on CNET. If you want the quick overview - this is the best place for it. the following quote even helped in purchasing my TV and saved me about £200 in the process!
"...Today, however, the premium for 1080p is still pretty steep, and unless you're
getting a very large set, say 50 inches or more, we don't recommend basing a
buying decision on whether or not the television has 1080p native resolution."
- David Katzmaier
1080i vs. 1080p HDTV: should you care?
Not for a while apparently - a slightly older article but from the looks of it and the way things are going...this still holds quite a bit of water.
All in all, unless your intention is to pay a fat wedge now so you won't have to later...then go for 1080p - but if for the next couple of years you're content to have a TV that looks pretty dope compared to that giant box you stare at that takes up space in your living room - 720p should be alright for you.