margin-left或者margin-right是怎么算的？和普通的left,right有什么區別？拜謝告知！

代碼如下：

#wrap{width:970px; margin:0 auto;}
#mainbody{position:relative; margin-top:15px;}
#left{float:left;width:110px;border:1px solid #999;}
#mid{margin-left:124px;width:650px;border:1px solid #999;}/*0+2+0+109+13*/
#right{position:absolute;top:0px;margin-left:792px;border:1px solid #999;}/*124+2+0+649+17*/

<body>
<div id="wrap">
???? <div id="header"><img src="http://img1.sycdn.imooc.com//5369cd6e0001a15b09700088.jpg" width="970" height="88" /></div>
???? <div id="mainbody">
????????? <div id="left"><img src="http://img1.sycdn.imooc.com//5369cd0e00011e3901090487.jpg" width="109" height="487" /></div>
????????? <div id="mid"><img src="http://img1.sycdn.imooc.com//5369cd3c00013e9e06490439.jpg" width="649" height="439" /></div>
????????? <div id="right"><p>歡迎使用金山軟件出品的愛詞霸，在線查詢你英語詞匯、句子釋義</p><img src="http://img1.sycdn.imooc.com//5369cd540001d8e101770329.jpg" width="177" height="329" /></div>
? </div>
</div>
</body>