I have asked this question myself, and been asked this question so many times I figured I would try to put an article together to answer the question.
With a quick Google search to began my research I found the most detailed and complete explanation had already been compiled. So to keep it easy (especially for me) I am just going to add a link to this excellent article.
http://www.luckygunner.com/labs/5-56-vs-223/
