What first guy said:
'A blend of electronica and down-tempo hip-hop, urban and ethereal, street and ambience. Thought provoking, sensual, and deep. Often features string section and/or brass section. Can be either instrumental or with female vocals. Occasionally with rap. Trancy and smooth. Often jazzy. Sometimes features turntablism / scratching. It's head nodding music. "Chill-out music".'
Names associated with this genre:
Kid Koala, Dan the Automater, Prince Paul, Del the Funkee Homosapien...etc
If you want a real trip-hop experience, check out Lovage's "Music to Make Love to Your Old Lady By"
Absolute musical ecstasy to the ears.
Also check out Gorillaz, Deltron 3030, Handsome Boy Modeling School, and Goldfrapp.