Queso fresco is a fresh, mild cow's-milk cheese used across Mexico. It crumbles. It is lightly salty and not aged.
In US shops it is the cheese you scatter on a finished dish. It softens when heated but does not become a smooth melt. Queso blanco is the name often used for a fresh cheese that holds its shape better in a pan.
History
Fresh cheese is made wherever there is milk and a reason to eat it this week. In Mexico, queso fresco became the ordinary crumbling cheese of the table and the taco stand. The name means fresh cheese, and it describes an age and a texture more than a single town. Different states have their own fresh cheeses with tighter local names. Queso fresco is the general word.
It traveled into US groceries with Mexican ingredients, often in a round plastic tub or a small wrapped disk. Cooks there use it to finish plates the way the cheese is used in Mexico: a handful over hot food, not a wheel on a board. It has no great export appellation and no aging houses. Its virtue is that it stays mild and looks like snow on a dark dish.
How it is made
Cow's milk is set, the curd is lightly salted, and it is drained just long enough to hold a shape and still crumble. It is not pressed for months and not stretched. It has no rind. Some versions are a little firmer than others. None of them is a keeping cheese. The date on the package matters more than a vintage.
The crumble is the design. If you fry a slab, it often sticks, browns in patches, and falls apart. That is why recipes tell you to add it after the heat has done its work. Queso blanco, in the US shopping sense, is the fresh cheese that can survive the pan. The two names are confused on labels often enough that you should read the suggested use, or look at the texture, before you cook.
Taste
Queso fresco tastes of fresh milk and a little salt, with a soft, moist crumb. It is slightly tangy when it is very fresh, and flat when it is old. It should not taste sour, bitter, or like blue cheese. There is no rind flavor and no smoke unless someone added them.
A dry, yellow, or sour disk has been kept too long. A rubbery disk that will not crumble may be a firmer fresh cheese, useful in its own way and wrong for a recipe that says to sprinkle. Taste it plain. If you taste almost nothing, that is normal. The food underneath is supposed to be louder.
In the kitchen
Crumble it over tacos, enchiladas, refried beans, soups, salads, and scrambled eggs after they leave the heat. It cools a spicy bite and adds salt and moisture. It can go inside a chile relleno or a quesadilla mixed with a melting cheese, but alone it will not stretch.
Do not fry it as the cheese in a grilled slab and expect halloumi. Do not melt it across the top of a casserole and expect Chihuahua or Jack. Cotija is the substitute when you want a saltier, drier finish. Queso blanco is the substitute when the recipe then says to brown the cheese in a pan. Mozzarella is the substitute only for stretch, and it will taste more elastic and less like fresh crumbs.
Around the world
Mexico is the reference, and the cheese is used in home kitchens and street food every day. Similar fresh cow's-milk cheeses exist all over Latin America under names that also translate as fresh or white. They are cousins. A recipe written in Mexico or in a Mexican-American kitchen means this crumbly finish.
In the United States the disk in the dairy case is now a normal ingredient, not an import reserved for a special trip. Buy it the week you will use it. When a recipe from another country says queso fresco, check whether it wants a crumble or a cheese that grills. The Spanish words alone are ambiguous. The Mexican meaning on this page is the mild crumble.