"on order" probably means we do accept your order but we didn't specify if the product is currently in stock or not. It usually used in pre-order product I think....actually I seldom see this term though.
"back order" then specifically states that we don't have stock presently and you still can order from us. Once we have it, we ship. (usually indicating roughly shipping date with it)
"out of stock" usually means we don't have that product and currently we are not ordering it from our suppliers.

小弟英文不算好 說起話來只能大概讓洋人聽得懂 至少當TA還OK
而且實在很難跟他們聊天 因為文化不同 興趣幾乎都沒有一樣 笑點也不同啊...
所以多半聊的是專業領域比較多 偶爾NBA可以講一下