Shredded bark (or bark dust) is made of common local tree bark (ie. cedar in California, Douglas fir in Oregon etc.) in varying sizes, which is used primarily as a decorative ground cover to retard weed growth.

Eventually, mulch breaks down to become part of the soil. Humus or black humus, for instance, is a decomposed wood product used primarily for soil amendment or improvement. You might find other types of mulch in your area.

-- Tips courtesy of HomeAdvisor.com