Parody music, or musical parodies, are comic or absurd representations of existing musical forms such as songs or videos. Parody of music has long existed, and in the 20th century it emerged as a catagory of music unto itself.
Pre-20th century
Even traditional ballads such as "Frére Jacques" have innumerable parodies written about them. Even Mozarts contemporaries were parodied in the late 18th century.
1900-1950
In the 1940s, Spike Jones and his City Slickers parodied popular music in their own unique way, not by changing lyrics, but adding wild sound effects and comedic stylings to formerly staid old songs such as "Cocktails for Two" and "April Showers."
1960-1980
Stan Freberg created parodies of popular songs in the 1950s and 1960s, mocking the musical conventions of the day, such as Elvis Presley's "Heartbreak Hotel" in which the vocalist rips his jeans from too much hip-swiveling and drowns in reverberative sound effects at the end. Another major parodist was Allan Sherman, who began making hit records with parodies such as the now-classic "Hello Mudduh, Hello Fadduh (A Letter from Camp)" (to the tune of Amilcare Ponchielli's "Dance of the Hours" from the opera La Gioconda) and "When I Was A Lad" (after Gilbert & Sullivan's "Ruler of the Queen's Navee" from "HMS Pinafore"). Yet another parodist was the self-described "piano-wielding fugitive from Harvard" named Tom Lehrer who created a group known as the Capitol Steps. There were a group of current and former U.S. Congress staff members based in Washington, DC which began recording and performing song parodies ridiculing politicians, public figures, fads and current events, and still do so today, performing on National Public Radio and at concert halls and other venues nationwide.
1980-Present
The most successful parodist of popular music is "Weird Al" Yankovic, now in his third decade of writing song parodies. He got his start sending tapes to be played by Barret Hansen, AKA Dr. Demento, on his nationally syndicated radio show. Seattle, WA-based disc jockey and longtime parodist Bob Rivers also records parodies of hit songs from a variety of genres and periods satirizing current events. Also dabbling in topical parodies is Buffalo, New York-based humorist Mark Russell, who appears several times a year on PBS television. The New York, NY performing troupe Forbidden Broadway annually parodies the Great White Way's most popular current musicals and their songs on stage and recordings.
And in the science fiction fan community of today, filk music thrives as a source of both parodies and original music, as it has since at least the 1930s, with artists such as Leslie Fish, Tom Smith (filker) and Frank Hayes gleefully adapting tunes from many genres to their own varied interests.
Even original artists of the caliber of Tom Lehrer have been known to dip a toe in the waters of musical parody; Lehrer's famous song "The Elements" adapts a tune from Gilbert & Sullivan to the periodic table, and more recently he turned "That's Entertainment" into a précis on his real vocation, "That's Mathematics" (carefully altering the melody to avoid litigation). Other well-known parody artists include Sheb Wooley, Tim Cavanagh, Flanders & Swann, Da Yoopers, Pinkard & Bowden and Carla Ulbrich.