Ground covers are as varied as any other category, so here are some guiding thoughts.  When we think of ground covers, certainly grass, maybe ivy or periwinkle come to mind.  While these herbaceous plants do indeed cover the ground, there are woody plants and vines that perform similarly.  Some examples include bougainvillea, familiar as a vine, is available in dwarf species.  When planted together act as a ground cover.  Junipers are popular drought tolerant ground covers, and depending upon the species, can spread within inches of the ground or several feet.  Carpet roses, as the name suggests, spread low to the ground, mounding, but essentially act as ground covers.  In other words, ground covers are typically any plants that spread horizontally wider than they are tall, providing ample coverage over exposed soils.  Depending on site conditions and design requirements, numerous options can be specified.