Toasting, Chatting, or Deejaying is the act of talking or chanting over a rhythm or beat. The lyrics can be either improvised or pre-written. Toasting has been used in various African traditions, such as griots chanting over a drum beat, Jamaican music forms, such as dancehall, reggae, ska, dub, and lovers rock. Toasting's mix of talking and chanting may have influenced the development of rapping in US hip hop music. The combination of singing and toasting is known as singjaying.
In the late 1960s and early 1970s a strain of Jamaican music called deejay toasting was developed. Deejays working for producers would play the latest hits on traveling sound systems at parties and add their "toasts" or vocals to the music. These "toasts" consisted of boastful commentaries, chants, half-sung rhymes, rhythmic chants, squeals, screams, and rhymed storytelling.
Osbourne Ruddock (aka King Tubby) was a Jamaican sound recording engineer who created vocal-less rhythm backing tracks that were used by DJs doing "toasting" by creating one-off vinyl discs (also known as dub plates) of songs without the vocals and adding echo and sound effects.
Late 1960s toasting deejays included U-Roy and Dennis Alcapone, the latter known for mixing gangster talk with humor in his toasting. In the early 1970s, toasting deejays included I-Roy (his nickname is a homage to U-Roy) and Dillinger, the latter known for his humorous toasting style. In the late 1970s, Trinity became a popular toasting deejay.
The 1980s saw the first deejay Toasting duo, Michigan & Smiley, and the development of toasting outside of Jamaica. In England, Pato Banton explored his Caribbean roots humorous and political toasting and Ranking Roger of the "Second Wave" or Two-Tone ska revival band The Beat from the 1980s did Jamaican toasting over music that blended ska, pop, and some punk influences.
The rhythmic rhyming of vocals in Jamaican deejay toasting influenced the development of rapping in African-American hip-hop, and the development of the Dancehall style. (e.g. hip-hop pioneer and Jamaican ex-patriate DJ Kool Herc and Phife Dawg of A Tribe Called Quest). Jamaican deejay toasting also influenced various types of dance music, such as jungle music, UK garage, and reggaeton. Dancehall artists that have achieved pop hits with toasting-influenced vocals include Shabba Ranks, Shaggy and Sean Paul.
Taken From: http://en.wikipedia.org/wiki/Toasting