A painting ground is the surface onto which you paint. It can be anything.
It is usually on top of a sealant/sizing layer of the surface.
To be structurally sound it should be compatible with both the underlying support and the paint that is going onto it. Just a reminder that an artist concerned with the permanence of his/her paintings should be as concerned with the proper preparation of the foundation layers of the painting that are perhaps not visible (the support, the size and the ground) as the layers they do see (the paint, mediums and varnish).
The ground is required both to give a suitable surface texture and also to give an opaque colour, to cover the canvas or panel colour with white or a tinted ground, or occasionally a dark colour.
Acrylic primer (less correctly called acrylic gesso) is an example of a ground that is also a size. It does both jobs, sealing the substrate and providing a good surface on which to paint oils or acrylics.
Genuine gesso is a painting ground for oil paint and egg tempera in particular, but any paint can be used on it.
An oil ground is oil paint painted on top of a sizing over the entire surface to prepare the surface for painting with oil colours. So using an oil primer means you cannot paint on that ground with acrylics as the ground will repel the paint.
Painting with soft pastels requires a ground with a tooth to pick up and hold the pigment particles. This toothy pastel ground can be painted onto paper, canvas or panels, or surfaces can be purchased with the ground already applied to them.
To create an absorbant paper-like surface on canvas or panels for painting with watercolours, Absorbant Ground can be used. It is painted onto sized or primed canvas or panels. It is the ground, not the size and the substrate must be sealed first.