Mulch consists of leaves, grass, twigs and any plant material that has dried. It can be used for very dry flowerbeds in order to keep the water in and once it “rots” away it serves as a compost.
Wood chips can also be used as a mulch or to keep moist in the soil or even for decoration in the flower bed.
Bark is used not only for mulching or to keep water in, but mainly for landscaping visualisation purposes.