Deep house is a subgenre of house music that originated in the 1980s, initially fusing elements of Chicago house with 1980s jazz-funk and touches of soul music.
Deep house is known for complex melody, hard driving beats, complex chords underlying most sequences, and a soul, ambient, or lounge vibe to the vocals (if any). In the early compositions (1988—89), influences of jazz music were most frequently brought out by using more complex chords than simple triads (7ths, 9ths, 13ths, suspensions, alterations) which are held for many bars and give compositions a slightly dissonant feel. The use of vocals became more common in deep house than in many other forms of house music. Sonic qualities include soulful vocals (if vocals are included), slow and concentrated dissonant melodies, smooth, stylish, and chic demeanor. Deep house music rarely reaches a climax, but lingers on as a comfortable relaxing sound.
Deep house was largely pioneered by Larry Heard (Mr. Fingers) with tracks such as "Mystery of Love" (1985) and "Can You Feel It?" (1986); the latter had a similar impact on deep house as Derrick May's "Strings Of Life" (1987) did on Detroit techno. Heard's deep house sound moved house music away from its "posthuman tendencies back towards the lush" soulful sound of early disco music (particularly that of old Philadelphia International and Salsoul records).
Later deep house tracks (1993—94) were more heavily influenced by disco and even merged into a disputable disco house genre. Modern deep house (post-2000) often shares features with the related genre of tech house but tends to focus on musical complexity where tech house focuses on simplicity.
...more info at http://en.wikipedia.org/wiki/Deep_house