跪求大侠帮我讲解一下C#连数据库的下面这段代码

Question

 public static DataRow LoginInfo;
        public static bool isManager;
        public static void CheckUsers(string UserID, string Pwd)
        {
            SqlConnection cn = new SqlConnection(Properties.Settings.Default.XkConnectionString);
            SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Users WHERE UserID=@UserID AND Pwd=@Pwd",cn);
            da.SelectCommand.Parameters.Add("@UserID", SqlDbType.NVarChar, 8).Value = UserID;
            da.SelectCommand.Parameters.Add("@Pwd", SqlDbType.NVarChar, 8).Value = Pwd;
            DataSet ds = new DataSet();
            da.Fill(ds);
            if (ds.Tables[0].Rows.Count > 0)
            {
                LoginInfo = ds.Tables[0].Rows[0];
                isManager = true;
            }
            else
                LoginInfo = null;
        }

Answer 1

public static DataRow LoginInfo;
        public static bool isManager;
        public static void CheckUsers(string UserID, string Pwd)
        {
//声明一个数据库连接，连接字符串从设置中的XkConnectionString取得
            SqlConnection cn = new SqlConnection(Properties.Settings.Default.XkConnectionString);
            SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Users WHERE UserID=@UserID AND Pwd=@Pwd",cn);
//下面对sql中的参数赋值
            da.SelectCommand.Parameters.Add("@UserID", SqlDbType.NVarChar, 8).Value = UserID;
            da.SelectCommand.Parameters.Add("@Pwd", SqlDbType.NVarChar, 8).Value = Pwd;
            DataSet ds = new DataSet();
//取得数据集，填充到ds
            da.Fill(ds);
            if (ds.Tables[0].Rows.Count > 0)
            {
                LoginInfo = ds.Tables[0].Rows[0];
                isManager = true;
            }
            else
                LoginInfo = null;
        }

Answer 2

da.SelectCommand.Parameters.Add("@UserID", SqlDbType.NVarChar, 8).Value = UserID;这是指哪个数据啊？
da.SelectCommand.Parameters.Add("@Pwd", SqlDbType.NVarChar, 8).Value = Pwd;
 

Answer 3

("SELECT * FROM Users WHERE UserID=@UserID AND Pwd=@Pwd",cn);中UserID=@UserID前面和后面的分别是哪里的参数啊？大侠