1. Some specific  name: XmlDocument XmlNodeList XmlNode XmlElement
    The whole architecture and relation among them is following
    Attentions:selectsinglenodelist（”users”）,用于获取users下面的所有直接子节点。这个方法经常用到
2. the difference between xmlnode and xmlelement
    XmlElement从XmlNode继承而来，功能基本相同，但XmlElement有更多的功能   
 3. Insert data into XML file sample:
  Sample 1:
  XmlDocument xdoc = new XmlDocument();
        xdoc.Load(HttpContext.Current.Server.MapPath("user.config"));

XmlElement xde = xdoc.DocumentElement;
        XmlElement xe = xdoc.CreateElement("user");
        xe.SetAttribute("Login", login);
        xe.SetAttribute("Password", pwd);
        xe.SetAttribute("RootFolder", rootfolder);
        xe.SetAttribute("Rights", rights);
        xde.AppendChild(xe);
        xdoc.Save(HttpContext.Current.Server.MapPath("user.config"));
the xml file is the following
<?xml version="1.0" standalone="yes"?>
<users>
  <user Login="sanxumei" Password="sanxumei" RootFolder="c:\inetpub\wwroot" Rights="User" />
</users>

Sample 2:
XmlDocument xmlDoc=new XmlDocument();
   xmlDoc.Load("bookstore.xml");
   XmlNode root=xmlDoc.SelectSingleNode("bookstore");//查找<bookstore>
   XmlElement xe1=xmlDoc.CreateElement("book");//创建一个<book>节点
   xe1.SetAttribute("genre","李赞红");//设置该节点genre属性
   xe1.SetAttribute("ISBN","2-3631-4");//设置该节点ISBN属性
 
   XmlElement xesub1=xmlDoc.CreateElement("title");
   xesub1.InnerText="CS从入门到精通";//设置文本节点
   xe1.AppendChild(xesub1);//添加到<book>节点中
   XmlElement xesub2=xmlDoc.CreateElement("author");
   xesub2.InnerText="候捷";
   xe1.AppendChild(xesub2);
   XmlElement xesub3=xmlDoc.CreateElement("price");
   xesub3.InnerText="58.3";
   xe1.AppendChild(xesub3);
 
   root.AppendChild(xe1);//添加到<bookstore>节点中
   xmlDoc.Save("bookstore.xml");