Song is the so-called "low cost" carrier within Delta. The aircraft are Delta 757s that have been re-branded as "Song". The company is owned by Delta, the pilots are Delta pilots operating under the same contract as any other Delta pilot, but the other employees are "employed" by Song.
It is nothing like the Air Tran/Ryan thing. These are two totally separate companies that have a wet-lease agreement. Delta and Song are the same company.