I have designed and programed many of websites by Dreamwaver 8, I see thre are many of differences between IE&FF: font size, table, css... there are many of things that are not understood by IE. Why? Some one can tell me why?

It's not that IE doesn't UNDERSTAND those things, it's that IE6 and below, specifically, are the LEAST standards-complaint and they are riddled with BUGS. Read the laundry list here:
http://www.positioniseverything.net/explorer.html
and here:
http://www.positioniseverything.net/ie-primer.html

IE7 is more standards compliant than it's predecessors, but it is not bug-free and it still has some of MS's NON-STANDARD behavior.