That is a great question.
There is a great explanation for these architectures (processor types) on wikipedia:
Yeah I tried their first, but X64 and IA64 now lead to the same page. I was looking for a simpler explanation without having to read a gigantic article.
read this http://mikedimmick.blogspot.com/2006/03/whats-difference-between-x64-and-ia-64.html
i hope it helps.