The general term is given to plants that cover the ground for the benefit of erosion control, temperature and moisture buffering and amenity.
In ornamental horticulture, we think of ground cover as that layer which is under a tree or over a difficult bank but the term has much wider connotations in native pasture.
It's the herbaceous layer, the low growing vegetation that carpets the planet and is surprisingly diverse.
But let's stick to the term as it is used in gardening...
Ground covers are the plants that form the lowest layer, an accompaniment to shrubs and trees. Ground covers fill that niche where the other forms don't.
They spread horizontally like Variegated Star Jasmine or Juniperus horizontalis do or multiply sideways like dwarf Bamboo. Other times, the plant species will be self seeding and produce offspring adjacent but the feature is that the plant is low and spreads.
Ground covers can be used to great effect when placed as a single species under trees where there is no shrub layer. It creates a layered effect that allows vision though to another desired vista. When designed this way it also addresses surveillance and safety concerns.
When you have ground covers you minimise erosion. Rain drops hit the ground with considerable force and etch away at the soil surface, but plants diminish that impact.
Weeds have more difficulty establishing when ground covers prevent light from reaching young weeds.
Of great benefit are ground covers that cascade over steep banks where access is limited.
Grass, vines and herbs may all be classified as ground cover. Find one that suits your position!