The layers of a painting are (from bottom) auxiliary support, support, size, ground, priming, and paint. Each layer has its own function and purpose.
Auxillary Support is the device or framework to which the support is fastened. An example of auxiliary support is the stretcher bars used to secure canvas.
Support is the surface or substrate to which the paint is applied. The support can be flexible or rigid; it's usually made of materials such as canvas, linen, wooden panels, boards, metal, and paper. The role of the support is to hold the paint film in place.
Size is a layer of hyde glue, acrylic polymer, or gelatine applied to the support; it prevents the support from having direct contact with the ground or paint. Size is used to protect the support from the deleterious effects of the ground or paint in oil-based media. Size is not usually applied to supports that hold water-soluble media with the exception of watercolours. Manufacturers size watercolour papers externally with gelatin/starch or internally with Aquapel (a synthetic glue).
Ground is a layer made of white pigment (colour), gypsum (chalk), and a binder (adhesive) applied to the support (or on top of the sizing) to receive and hold the paint. The ground must have the required absorbency, porosity, and roughness. The ground can be water-based or oil-based. Water-based grounds can be used for oil painting but not the other way around. There is a school of thought that considers water-based grounds should only be used for water-soluble media.
Priming is applying a layer of diluted polymer medium (for water-soluble media) or diluted retouch varnish (for oil-based media) to the ground. Priming reduces the absorbency of the ground and enables better manipulation of paint.
Paint is the medium used to create the painting. Examples of media are oils, acrylics, watercolours, gouache, egg tempera, and casein.
(Note: Priming should not be mistaken for the application of ground. The confusion arises because the terms 'Primer' and 'Prime' are commonly used in the product descriptions to refer to ground.)