这是我ASP。net 实训的作品！我有些地方不明白，想问下高手！小弟感激不尽！

小弟我有如下代码，有些地方不带懂，就是红色字体那些，希望看到的高手能解释一下
public partial class latecheck : System.Web.UI.Page
{
    DB db = new DB();
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)//页面不回传，页面初次加载时则加载系部信息
        {
            string strSql = "select * from department";
            DataTable dt = db.reDt(strSql);
            xibuList.DataSource = dt;
            xibuList.DataTextField = "dept_name";
            xibuList.DataValueField = "dept_id";
            xibuList.DataBind();
            xibuList.Items.Insert(0, new ListItem("==请选择==", ""));
            classList.Items.Insert(0, new ListItem("==请选择==", ""));
            zhuanyeList.Items.Insert(0, new ListItem("==请选择==", ""));
            
        }

    }
    protected void xibuList_SelectedIndexChanged(object sender, EventArgs e)
    {
        string dept_id = xibuList.SelectedIndex.ToString();
        if (dept_id != "")//系部选择不为空时加载专业信息
        {
            string strSql = "select * from special where dept_id="+dept_id;
            DataTable dt = db.reDt(strSql);
            zhuanyeList.DataSource = dt;
            zhuanyeList.DataTextField = "spc_name";
            zhuanyeList.DataValueField = "spc_id";
            zhuanyeList.DataBind();
            zhuanyeList.Items.Insert(0, new ListItem("==请选择==",""));

         
        }
        else
        {
            zhuanyeList.Items.Clear();
            zhuanyeList.Items.Insert(0, new ListItem("==请选择==",""));
        }
    }
    protected void zhuanyeList_SelectedIndexChanged(object sender, EventArgs e)
    {
        
        string spc_id = Request.Form["zhuanyeList"];//这里为什么要获取集合而不用 string spc_id= zhuanyeList.SelectedIndex.ToString();呢？
        if (spc_id != "")//专业选择不为空时加载班级信息
        {
            string strSql = "select * from class where spc_id="+spc_id;
            DataTable dt = db.reDt(strSql);
            classList.DataSource = dt;
            classList.DataTextField = "class_name";
            classList.DataValueField = "class_id";
            classList.DataBind();
            classList.Items.Insert(0, new ListItem("==请选择==",""));
        }
        else
        {
            classList.Items.Clear();
            classList.Items.Insert(0,new ListItem("==请选择==",""));
        }

        
    }
}