张三丰28式太极拳:this.req.responseText是什么意思

var strText = this.req.responseText;这是页面文件中的<scrip>标签中的一句话
看到这么一句话，，那个this.req是什么？responseText又是如何定义的？是不是因为这一句中有什么语法含义，所以才能这么写？this.req是定义在外部js文件中的,整个js文档内容如下，请高手解释一下 this.req.responseText的语法含义和具体的相关知识,谢谢回答的高手
var net=new Object();

net.READY_STATE_UNINITIALIZED=0;
net.READY_STATE_LOADING=1;
net.READY_STATE_LOADED=2;
net.READY_STATE_INTERACTIVE=3;
net.READY_STATE_COMPLETE=4;

/*--- content loader object for cross-browser requests ---*/
net.ContentLoader=function(url,onload,onerror,method,params,contentType){
this.req=null;
net.currentLoader=this;
this.onload=onload;
this.onerror=(onerror) ? onerror : this.defaultError;
this.loadXMLDoc(url,method,params,contentType);
}

net.ContentLoader.prototype.loadXMLDoc=function(url,method,params,contentType){
if (!method){
method="GET";
}
if (!contentType && method=="POST"){
contentType='application/x-www-form-urlencoded';
}
if (window.XMLHttpRequest){
this.req=new XMLHttpRequest();
} else if (window.ActiveXObject){
this.req=new ActiveXObject("Microsoft.XMLHTTP");
}
if (this.req){
try{
var loader=this;
this.req.onreadystatechange=function(){
net.ContentLoader.onReadyState.call(loader);
}
this.req.open(method,url,true);
if (contentType){
this.req.setRequestHeader('Content-Type', contentType);
}
this.req.send(params);
}catch (err){
this.onerror.call(this);
}
}
}

net.ContentLoader.onReadyState=function(){
var req=this.req;
var ready=req.readyState;
try{var httpStatus=req.status;
if (ready==net.READY_STATE_COMPLETE){
if (httpStatus==200 || httpStatus==0){
this.onload.call(this);
}else{
this.onerror.call(this);
}
}
}
catch(e){}
}

net.ContentLoader.prototype.defaultError=function(){
alert("error fetching data!"
+"\n\nreadyState:"+this.req.readyState
+"\nstatus: "+this.req.status
+"\nheaders: "+this.req.getAllResponseHeaders());
}