Selector是利用css selector来匹配选择页面元素的，所以要理解Selector首先应该对css selector有所理解，下面是css2 selector的语法，当然很多浏览器只是支持其中的一部分，Prototype 中的Selector主要支持tag选择器、class选择器和id选择器，还有属性(attribute)选择器，基本上包含我们平时所用的所有类型
（本文来源于图老师网站，更多请访问https://m.tulaoshi.com/webkaifa/）The following table summarizes CSS2 selector syntax, 详细的可以看http://www.w3.org/TR/REC-CSS2/selector.html:
Pattern | Meaning | Described in section | *Matches any element.Universal selectorEMatches any E element (i.e., an element of type E).Type selectorsE FMatches any F element that is a descendant of an E element.Descendant selectorsE FMatches any F element that is a child of an element E.Child selectorsE:first-childMatches element E when E is the first child of its parent.The :first-child pseudo-classE:link E:visitedMatches element E if E is the source anchor of a hyperlink of which the target is not yet visited (:link) or already visited (:visited).The link pseudo-classesE:active E:hover E:focusMatches E during certain user actions.The dynamic pseudo-classesE:lang(c)Matches element of type E if it is in (human) language c (the document language specifies how language is determined).The :lang() pseudo-classE + FMatches any F element immediately preceded by an element E.Adjacent selectorsE[foo]Matches any E element with the “foo” attribute set (whatever the value).Attribute selectorsE[foo=”warning”]Matches any E element whose “foo” attribute value is exactly equal to “warning”.Attribute selectorsE[foo~=”warning”]Matches any E element whose “foo” attribute value is a list of space-separated values, one of which is exactly equal to “warning”.Attribute selectorsE[lang|=”en”]Matches any E element whose “lang” attribute has a hyphen-separated list of values beginning (from the left) with “en”.Attribute selectorsDIV.warningHTML only. The same as DIV[class~=”warning”].Class selectorsE#myidMatches any E element ID equal to “myid”.ID selectorsSelector中包含Selector对象和类，
Selector对象具有下面两个方法：
match(element)：元素是否与本selector匹配，在Element中已经介绍了
findElements(parentNode)：parentNode中所有匹配本selector的子孙元素列表
使用方法也很简单 var s=new Selector(expression); s.match(element); s.findElements($(element))，其中expression可以是如下方式 "div"、"#id"、".class"、"div#id"、"div[attribute]"、"div[attribute=fff]"、"div[attribute!=sdf]"
其中Selector也有几个静态方法，它们分别是：
matchElements(elements, expression)：返回elements中符合expression的元素列表
findElement(elements, expression, index)：返回elements中符合expression的元素列表中索引为index的元素
findChildElements(element, expressions)：找出element的子孙元素中符合expressions的元素列表，其中expressions是一个expression数组，其中的expression支持"div li.#id"形式
（本文来源于图老师网站，更多请访问https://m.tulaoshi.com/webkaifa/）$$方法：只是简单的调用return Selector.findChildElements(document, $A(arguments))
虽然Selector有这么多方法，但是大部分都是内部调用的，我们一般都很少使用，因为我们有个一个方便的方法$$，对于绝大部分情况已经足够了 
