Southern Gospel
Southern Gospel is a form of Christian music which, as its name suggests, was popularized in the Southern United States. Debate continues on when it first became commercial but it can trace back to at least 1910 (when the Stamps-Baxter Music Company hired a quartet to sell its music). Because of its beginnings in white churches it has different roots than black gospel.
It is commonly known as "quartet music" due to its traditional arrangement of four male singers (in a baritone-lead-tenor-bass arrangement) along with (most commonly) a pianist. Although that is still the most common arrangement, trios, larger groups, and family groups (with female singers) have gained popularity.
Southern Gospel grew from regional to national and even international prominence with the assistance of Bill and Gloria Gaither, a Midwestern couple who started their own group (the Gaither Vocal Band) and produced numerous music videos of their concerts. It has also gained small popularity outside of Anglo circles.
Due to its overlap with country music (in terms of origins) many country singers either came out of Southern Gospel (e.g. The Statler Brothers and The Oak Ridge Boys) or have released gospel albums themselves.
The major differences between Southern Gospel and Contemporary Christian Music are in the words: Southern Gospel lyrics are usually sung about God and are overt, while CCM lyrics are usually sung to God and often contain metaphors.