A style of music originating in the '80s, mainly through the innovations of the Red Hot Chili Peppers. The bassist uses a slap bass technique, incorporating the strong semiquaver rhythms and popping basslines associated with '70s funk music. While funk drew heavily from black soul music, funk-metal utilised distorted guitars and more prominent drumming. Combined with grittier rock-style vocals, it led to an aggressive metallic sound.
Funk-metal is used to describe quite diverse and eclectic acts, so the similarities may be unclear initially. The Red Hot Chili Peppers used thin guitar tones and atonal "rap" vocals to highlight the funky ambience of the bass and drums. Fishbone added ska rhythms and a horn section to the mix. Faith No More pushed the funk aspects of their sound further down the mix; this allowed the thick guitar sound of Jim Martin and distinctive vocals of Mike Patton to dominate. Softer alternative rock bands like Jane's Addiction and Incubus also often incorporated funk-metal into their sound.
Because funk-metal doesn't always work well commercially, bands like RHCP and Incubus have tended to soften their sound with later releases. I offer no criticism, but if you wish to experience true funk-metal, it would be wise to ignore recent releases. In the case of RHCP, their last extensive offering of funk-metal was Blood Sugar Sex Magik (1991).
Some good examples of funk-metal songs (to help you to understand what the sound is about) include:
N.B. I am not trying to impart any subjective view of quality on the above list. I selected the songs because they strongly demonstrate the various traits of funk-metal music, not because I like them.