cows are ruminant animals
A cows stomach is complex consisting of the Omasum, Abomasum, Reticulum and Rumen when the grass is digested it goes through this four stomach and it goes through the Rumen which has millions of microbes which aid in the digestion of these grasses .Mammals capable of eating grass are able to do so because of the action of a range of symbiotic microbes that live in the digestive track and have the ability to break down the cellulose (as well as the protein, starch, and fats) in the grass into forms available to the digestive functions of the host animal.
In the human body, we have a simple stomach which digest the food eaten.