Masa & Masa Harina
Though technically the word means "dough" in Spanish, in Mexico it is generally understood to be "corn dough", the ubiquitous dough of tortillas and tamales.
Fresh masa: 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.
Masa can vary from white to yellow, depending on hte type of corn used. Either is good, but the taste will be slightly different.
For tortillas, the fresh dough should be wrapped, refrigerated and used within 1 day. If using within a few hours, there is no need to refrigerate. Even before the masa spoils (3 or 4 days), it will lose the plasticity crucial to making light tortillas.
Masa for tamales can be frozen for a month or more, well wrapped. When a recipes calls for fresh masa, the smooth-ground masa for tortillas should be used, unless otherwise indicated.
Masa Harina: This powdery-looking meal, sold in many chain supermarkets and all Mexican groceries, is the fresh corn masa that has been dried and then powdered. It is not the same as fine-ground corn meal, so don't try to substitute! Masa harina is certainly more readily available to the average cook than the quick-perishing fresh masa, but the flavor is a little different.
The Quaker Oats brand is most common, but many Mexican groceries in the United States also carry the Maseca brand (Mexican brand) or at least in any grocery store with a decent-size "international" aisle. Stored in a dry place and wrapped well, it will last a year or so.
"Masa harina for tamales" should be used for tamales if you can't find fresh masa. The only brand in the United States is Maseca, and it is called "masa instantánea de maiz para hacer tamales" or "instant corn masa mix for tamales". It produces a beautiful light texture, though it lacks some of the earthy sweetness and toothsome textural irregularities of fresh coarse-ground masa.
Making tamales from masa harina for tortillas works too but the tamales will be a little less flavorful, texturally uniform and a little denser. Rest assured the tamales are still plenty acceptable.