Experimental, avantgardist music. Industrial music emerged in the mid-1970s and stems from the idea of music created for a new generation, with previous music being more "agricultural". Even though artists preferred electronic equipment, traditional instruments were sometimes still used, but usually in nontraditional ways. Industrial artists will often pick difficult or taboo themes that will reflect both musically and visually in their output.
Post-Industrial is a collection of styles that emerged in the early 1980s, all of which incorporate Industrial music with various other styles, resulting in for example Heavy Electronics, Dark Ambient (Ambient Industrial), Martial Industrial, etc.. Many of these styles have their own entry on Discogs.