（3）选择元素——（2）文档对象模型（The Document Object Model）
One of the most powerful aspects of jQuery is its ability to make selecting elements in the DOM easy. The Document Object Model serves as the interface between JavaScript and a web page; it provides a representation of the source HTML as a network of objects rather than as plain text.
jquery最有力的能力之一是它使选择DOM结构中元素的变的容易的能力。文档对象模型作为js和网页的接口存在，他提供了基础html作为对象网而不是简单的文本的存在。
This network takes the form of a "family tree" of elements on the page. When we refer to the relationships that elements have with one another, we use the same terminology that we use when referring to family relationships: parents, children, and so on. A simple example can help us understand how the family tree metaphor applies to a document, as follows:
<html>
<head>
<title>the title</title>
</head>
<body>
<div>
<p>This is a paragraph.</p>
<p>This is another paragraph.</p>
<p>This is yet another paragraph.</p>
</div>
</body>
</html>
<head>
<title>the title</title>
</head>
<body>
<div>
<p>This is a paragraph.</p>
<p>This is another paragraph.</p>
<p>This is yet another paragraph.</p>
</div>
</body>
</html>
With this tree of elements at our disposal, we'll be able to use jQuery to efficiently locate any set of elements on the page. Our tools to achieve this are jQuery selectors and traversal methods.
An important point to note, before we begin, is that the resulting set of elements from selectors and methods is always wrapped in a jQuery object. These jQuery objects are very easy to work with when we want to actually do something with the things that we find on a page. We can easily bind eventsto these objects and add slick effectsto them, as well as chainmultiple modifications or effects together. Nevertheless, jQuery objects are different from regular DOM elements or node lists, and as such do not necessarily provide the same methods and properties for some tasks. In the final part of this chapter, therefore, we will look at ways to directly access the DOM elements that are wrapped in a jQuery object.