The terms queso fresco ("fresh cheese") and queso blanco ("white cheese") are often used interchangeably, and they are both mild, semi-soft, crumbly, unaged cow's milk cheeses. Queso fresco is made with rennet, an enzyme that curdles the milk. The curds are pressed slightly, and queso fresco will melt when heated. Queso blanco is made from milk that has been curdled with an acid, like lemon juice. Both cheeses are usually salted. Queso blanco does not melt well, but softens and becomes more creamy when heated. Queso blanco is often labeled as a "frying" cheese, or "queso para freir" because it can be sliced and heated without losing its shape.
Both queso fresco and queso blanco are commonly used in South American cooking, and it can be difficult to distinguish between the two. Different regions have their own versions, and may not distinguish the two types by name, as they are both "fresh" (unaged) white cheeses. If the word "enzyme" is included in the ingredient list, then rennet (rather than acid) was probably used to curdle the milk.
Both of these cheeses are used in salads, crumbled over rice and beans, and even used in sauces such as Peruvian huancaína sauce. You can easily make homemade queso blanco. This excellent instructional video demonstrates how to make fresh cow's milk cheese. Queso blanco is made exactly this way, except that more water is pressed out to give it a slightly firmer and more crumbly texture.