I can't improve on Wikipedia......
"Dub is a genre which grew out of reggae music in the 1960s. Music in this genre consists predominantly of instrumental remixes of existing recordings and is achieved by significantly manipulating and reshaping the recordings, usually by removing the vocals from an existing music piece, and emphasizing the drum and bass parts (this stripped-down track is sometimes referred to as a 'riddim'). Other techniques include dynamically adding extensive echo, reverb, panoramic delay, and occasional dubbing of vocal or instrumental snippets from the original version or other works."
