Corn tortillas are flattened round disks made from corn which has been soaked (slaked) in "cal". The soaked corn is referred to as nixtamal and is ground while moist to make a fine, slightly gritty flour. Water is added to the ground corn flour to form a soft dough. The tortilla is then flattened in a heavy tortilla press or by hand. If you are going to purchase corn tortillas look for a brand which only contains flour, water, lime. Good quality corn tortillas do not contain salt or any additives or preservatives.
Take a look at our article which provides with various tips for heating and storing corn tortillas.