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isolate and protect the support
from the actual paint application
and is a reflector of light.
Grounds can be oil-based or acrylic-based. 
Oil-based grounds are traditionally used with oil colors; however nowadays
the most commonly used grounds, both for oil and acrylic colors, are acrylic-based grounds.
Gesso is...
The common name used for acrylic-based grounds.
Gesso is the best ground used with acrylic colors.

ADVANTAGES
Gesso is permanently flexible.  Does not become brittle as it ages.
It sticks to almost any surface free of oil or grease.
Dries quickly
Never cracks
When dry, it completely resists water and moisture. Is nontoxic. Safe and clean. Inexpensive.

DISADVANTAGES
Gesso expands and contracts with changes in humidity; therefore, when used with oil colors on a canvas, this presents a problem because oil colors become harder and less flexible over time. When applied over an eternally flexible ground in time  oil colors will break and chip off the surface. 
However, this problem does not exist when used on a rigid support.