Masa is a dough made from corn flour and used to make tortillas, tamales, etc.
Masa harina is the traditional flour used to make tortillas, tamales, and other Mexican dishes. Literally translated from Spanish, it means "dough flour," because the flour is made from dried masa, a dough from specially treated corn.
To make masa harina, field corn (or maize) is dried and then treated in a solution of lime and water, also called slaked lime. This loosens the hulls from the kernels and softens the corn. In addition, the lime reacts with the corn so that the nutrient niacin can be assimilated by the digestive tract.The soaked maize is then washed, and the wet corn is ground into a dough, called masa. It is this fresh masa, when dried and powdered, that becomes masa harina. (Add water once again to make dough for tortillas or tamales).