The difference between posting a reply and the box at the bottom which says "quick reply"?

Great question. The quick reply is perfect if you dont want to add a lot of formatting options. It works exactly the same as hitting post reply, but you dont have a list of all the smiles and formating controls.

Saying that, you can still put in the shortcuts for the smiles and the formating codes and it then works exactly the same.

It is there so that you can quickly reply to a thread without having to load up another page.

well thats certainly a great option, I hardly ever use anything like that, being the pc illiterate I am