Funk, in its classic sense, was essentially a hybrid of soul and rock that borrowed cues from jazz and gospel as well. It was an aggressive form of music, and often conveyed a topical message. It is epitomized by the soundtracks to early 1970's blaxploitation movies, which were essentially gangster or cop movies set in the ghetto. It was an organic outgrowth of James Brown's aggressively danceable soul and George Clinton's warped, druggy Gospel and doo wop; it borrowed tempos and electric guitar pyrotechnics from rock, as in the case of Sly & The Family Stone. The Isley Brothers, an r&b combo dating back to "Shout" in the early 60's, reinvented itself as a slick, modern funk band and saw the most cash rich days of their lives. Miles Davis released On the Corner in 1972, an album that was considered blasphemous by jazz purists, but was embraced by the funk generation; its influence of funk-leaning artists of the 1980's, from Vernon Reid on down, was immense.
It is the form of black-identified music that is most closely related to rock; all of the sonic thrills one seeks in psychedelic music, heavy metal, or progressive rock are here. But it is still black music; the most convincing funk number by a white group was probably Wild Cherry's 1977 fluke "Play That Funky Music (White Boy)"
Funk was characterized by its tight rhythms and horn sections. Its lyric matter generally were either reports from the ghetto, or sleazy invitations to party. It never really gained the respectability of soul or r&b, but it is a vital music that still sounds fresh today.
Funk's heyday stretched from about 1969, when Sly & The Family Stone were at their peak, to around 1980, when disco had supplanted funk as dance music, and rap was on the horizon. However, funk never really disappeared; it has enjoyed a tremendous renaissance due to its appearance in movies, and much of its music has been sampled by electronica and trip-hop artists, electronica being the primary domain of contemporary funk. It continues to inform rap, hip-hop, and soul. Prince can dish out the real thing when he's in the mood. Lenny Kravitz and Red Hot Chili Peppers are other names among dozens who owe debt to funk. Tricky's 1995 Maxinquaye is a primo example of electronica that is also recognizable funk, in spirit and sound.