Traditional pop (also classic pop or pop standards) music consists of Western (and particularly American) popular music that generally predates the advent of rock and roll in the mid-1950s.
Traditional/classic pop music is generally regarded as having existed between the mid-1940s and mid-1950s. Allmusic defines traditional pop as "post-big band and pre-rock & roll pop music". This definition is disputed by many scholars, however, as many of the most enduring and popular standards predate World War II, as for example most of the work of Cole Porter and George and Ira Gershwin -- and in some cases, as with Irving Berlin and Jerome Kern, even World War I. The most popular and enduring songs from this style of music are known as pop standards or (where relevant) American standards. More generally, the term "standard" can be applied to any popular song that has become very widely known within mainstream culture.
Traditional pop music is often regarded as having enduring appeal, possessing certain ineffable qualities, including but not limited to an ease and memorability of melody, as well as wit and charm of lyric. The greatest of the classic pop songwriters achieved this with regularity. Many, if not most, of these songs and songwriters are part of the Great American Songbook.
Classic pop embraces the song output of the Broadway, Tin Pan Alley, and Hollywood show tune writers from approximately World War I to the 1950s, such as Irving Berlin, Victor Herbert, Harry Warren, Harold Arlen, Jerome Kern, George Gershwin and Ira Gershwin, Richard Rodgers, Lorenz Hart, Oscar Hammerstein, Johnny Mercer, Dorothy Fields, Hoagy Carmichael, Cole Porter and a host of others. The works of these songwriters and composers are usually considered part of the canon known as the "Great American Songbook".
The Swing Era is often regarded as the most influential precursor to traditional pop music because it contributed greatly to the nationwide popularity of classic American "pop standards".
Mid-1940s to mid-1950s: Height of popularity
Following the swing era, many of the bands and vocalists became part of the period's popular music. Two notable differences were the addition of string sections to many of these orchestras and more emphasis on the vocal performance. The addition of lush strings can be heard in much of the popular music throughout the 1940s and 1950s. Popular singers during this period include a young Frank Sinatra, Bing Crosby, Dinah Shore, Jo Stafford, Perry Como, Patti Page, etc.
Late 1950s to 1960s: Decline of traditional pop
In the late 1950s and 1960s, rock became a very prominent musical style. However, pop singers who had been popular during the swing era or traditional pop music period were still big stars (i.e. Frank Sinatra, Doris Day, Ella Fitzgerald, and Dinah Shore).
Some of these vocalists faded with traditional pop music, while many vocalists became involved in 1960s' vocal jazz and the rebirth of "swing music"; the swing music of the 1960s is sometimes referred to as easy listening and was, in essence, a rebirth of the swing music that had been popular during the swing era, however with more emphasis on the vocalist. Like the Swing Era, it too featured many songs of the Great American Songbook. Much of this music was made popular by Nelson Riddle, and television friendly singers like Rosemary Clooney, Dean Martin, and the cast of Your Hit Parade. Many artists made their mark with pop standards, particularly vocal jazz and pop singers like Ella Fitzgerald, Billie Holiday, Frank Sinatra, Doris Day, Frankie Laine, Nat King Cole (originally known for his jazz piano virtuosity), Lena Horne, Tony Bennett, Vic Damone, Johnny Mathis, Bobby Darin, Barbra Streisand, Peggy Lee, Sammy Davis, Jr., Mel Tormé, Sarah Vaughan, Eydie Gormé, Andy Williams, Nancy Wilson, Jack Jones, Rita Reys, Steve Lawrence and Cleo Laine.
It is also worth noting that in addition to the vocal jazz and/or 1960s swing music, many of these singers were involved in "less swinging", more traditional, vocal pop music during this period as well, namely Sinatra and Cole.
Advent of rock and roll
With the growing popularity of rock and roll in the 1950s, much of what baby boomers considered to be their parents' music, traditional pop, was pushed aside. Popular music sung by Frank Sinatra, Ella Fitzgerald, Peggy Lee and their contemporaries was relegated in the 1960s and 1970s to Las Vegas club acts and elevator music.
A major change in popular culture came in 1983 when singer Linda Ronstadt, then considered one of the leading female vocalists of the rock era elected to change the direction of her career. She collaborated with legendary orchestra leader Nelson Riddle and released a hugely successful album of standards from the 1940s and 1950s, What's New. It reached #3 on the Billboard pop chart, won a Grammy, and inspired Ronstadt to team up with Riddle for two more albums: 1984's Lush Life and 1986's For Sentimental Reasons. The gamble paid off, as all three albums became hits, the international concert tours were a success and Riddle picked up a few more Grammys in the process. Ronstadt's courage and determination to produce these albums exposed a whole new generation to the sounds of the pre-swing and swing eras.
Using the Ronstadt prototype, rock/pop stars singing traditional pop music for a large commercial market has become accepted and bankable. Some examples include Cyndi Lauper, Sheena Easton, Queen Latifah, Willie Nelson, Fiona Apple, Joan Osborne, Rita Coolidge and Rod Stewart, all of whom have made forays into this once-shunned territory.
In recent times, there appears to have been a union of rock n roll with traditional pop, as many current pop stars and musicians use rock and roll instrumentation but with arrangements and compositions in the spirit of predecessors from the earlier era. An example of this is vocalist Michael Bublé's interpretation of The Beatles' rock and roll hit, "Can't Buy Me Love", performed in more traditional pop arrangement.
Current adherence to traditional pop
The appearance of lounge subculture in the mid-1990s in the United States helped to enhance the revival and interest in the music, style, and performers of popular music prior to rock and roll. Many contemporary performers have worked in the style of classic pop and/or easy listening swing, including Harry Connick, Jr., Linda Ronstadt, Michael Bublé, Diana Krall, Stacey Kent, John Pizzarelli, Ray Reach, Karrin Allyson, Madeleine Peyroux, Jane Monheit, Maude Maggart, as well as those known as cabaret singers such as Andrea Marcovicci and Bobby Short.
At present, the history or historiography of traditional pop music is still a moving target, with vocalists and music fans continuing to appreciate these timeless songs. In recent years, Rod Stewart has concentrated on reintroducing the "Great American Songbook" to a large scale audience in the same manner Linda Ronstadt did twenty years prior. His first album from the songbook series, It Had to Be You... The Great American Songbook, reached #4 in the U.S. pop chart, and its success led him to release four more albums in this vein. These commercial successes show not only the fine craftsmanship behind the creation of these types of popular songs, but also the desire and enthusiasm the public has when presented with a great song and melody.
Singers and groups associated with traditional pop
The Andrews Sisters
The Boswell Sisters
The DeCastro Sisters
The Fontane Sisters
The King Sisters
The Lennon Sisters
The McGuire Sisters
(from the article Traditional pop of Wikipedia)