Mulch is the protective layer on top of the soil in your garden. It reduces erosion, increases water retention and adds the definitive "final touch" to the look of a fresh garden.
Wood mulch breaks down over time, adding nutrients to the soil as it does so. It needs to be refreshed with new mulch every year. Spring or Fall Cleans make easy times to establish your annual mulching scheduling.
Rock mulch moves with time. It needs annual raking to keep it looking its best. It eventually needs replenishment, but on a much longer timetable than wood mulches.