What's the difference between masa for tortillas and for tamales?
I found a link that will provide the explanation you are looking for: 

http://www.mymexicanrecipes.com/ingredients/masa-harina.html

Here is a direct quote from the site:


  Masa is dried corn that has been cooked in limewater (cal), soaked overnight, and then ground up while still wet. Sold in this form, it's called fresh masa, and it makes the lightest, fluffiest tamales. In Mexico, it can be purchased at tortilla factories in two ways: with smooth consistency for making tortillas and, upon request (if you are lucky), with a coarser consistency for making tamales. (Be careful to differentiate between masa para tamales and masa preparada para tamales; the latter, available at some tortilla factories and large Mexican groceries, is the coarse-textured masa mixed with lard and flavorings.