The term Enka (演歌) was originally used to refer political protest songs from the late 19th century; however, from the 1960s onwards, it has referred to Japanese popular music in a minor key, sung with no accompaniment, in a "melisma" style (where a single syllable of text is sung while moving between several different notes in succession). In Japanese, this vocal style is known as "kobushi". To put it more simply, it means that the pitch of the singer's voice fluctuates irregularly; this compares with vibrato, which vibrates in a perfectly regular cycle. Although the kobushi technique is most widely associated with enka, is not limited to enka, as it can also be heard in the Italian song "Santa Lucia."
The "kobushi" vocal style developed in the late 1930s, and was influenced by Scottish, Irish, Italian and Russian folk melodies, chanson and big band, as well as drawing on Japanese musical traditions. Koga Masao, a very popular and influential composer primarily active in the late 1930s and 40s, began to include Buddhist "shoumyou" chanting style vocals into his songs, because his record label asked him to produce more popular music; this shoumyou-type vocal is kobushi-like. As the nascent genre developed, however, the vocals moved away from shoumyou and more towards the "modern" type of kobushi.
The Western music scale has 7 notes, the well-known "Do-Re-Mi-Fa-So-La-Ti", but the native Japanese scale only has 5: "Do-Re-Mi-So-La". For this reason, the Japanese scale can be referred to as "ヨナ抜き" (Yona-nuki : "Yo" means 4, "Na" means 7, "Nuki" means "without"): it means that notes four and seven in the western scale are "removed".
Throughout the 1950s-1970s, artists like Misora Hibari, Kitajima Saburou, solidified enka's place in Japanese pop music (sometimes with songs that had to be retroactively labelled "enka"), but past the 1970s, enka's market share dropped off significantly, even as notable artists continued to make hits in the 80s and beyond.
Often confused for Enka are related genres like "盛り場ポップス" (sakariba pops / bar pops) and "ムード歌謡" (mood kayou). Some labels even tried to push these (often short-lived) subgenres as a marketing strategy. RCA and Nippon Crown were particularly known for this; RCA pushed Fuji Keiko, Uchiyamada Hiroshi & his Cool Five, and Kaji Meiko as "enka" artists, although strictly speaking, they were singing enka-influenced kayoukyoku songs. (Not that most people are particularly bothered.) Another similar genre is the nagashi-uta, a subgenre of songs with darker themes which is also influenced by traditional Japanese music; this made a resurgence with the popularity of folk-rock in the 1960s and 70s.
Other musically-related kanji that are pronounced "enka" occasionally confuse the naive reader as well. "艶歌" means "sexy, bewitching, voluptuous" and is occasionally used in easy-listening "pornogroove" type kayoukyoku albums (which are often enka-influenced, but usually lacking kobushi vocal style, or any vocals at all). "怨歌" is rarely used, but it means "resentment song"; the resentment song has very dark or negative themes, and is enka-styled, but may or may not fall under the formalistic enka umbrella. The "resentment song" was very popular during the 60s and 70s as an expression of the rage of Japanese youth against the Japan-US Security Treaty and the Vietnam War. A third set of kanji is "援歌" or "cheer song", which is generally enka-like, but sung in an upbeat major key and at a faster "marching" style tempo, and often with backup singers, like Suizenji Kiyoko's "三百六十五歩のマーチ".