What is the difference between null and System.DBNull.Value?

Question

Is there any difference between null and System.DBNull.Value? If yes, what is it?

I noticed this behavior now -

while (rdr.Read())
{
    if (rdr["Id"] != null) //if (rdr["Id"] != System.DBNull.Value)  
    {
        int x = Convert.ToInt32(rdr["Id"]);
    }
}

While I retrieve data from the database using a sql datareader, though there is no value returned if(rdr["Id"] != null) returned true and eventually threw an exception for casting a null as integer.

But, this if I use if (rdr["Id"] != System.DBNull.Value) returns false.

What's the difference between null and System.DBNull.Value?