Quesillo (pronounced "keh-SEE-yoh") is a mild string cheese which is produced in Oaxaca. Outside Oaxaca it is sometimes referred to as queso Oaxaca or queso de hebra. Quesillo is made with cow's milk. The production process involves stretching the cheese into long strips and then rolling it into a ball. The cheese is sold by weight. This type of cheese melts well and is perfect for making quesadillas or, as we'll see next, tlayudas.
Empanadas de quesillo con flor de calabaza (quesillo empanadas with squash blossoms), as in the above photo, are an ideal way to enjoy quesillo.
Queso fresco, a crumbly cheese, is the other type of cheese which is ubiquitous in Oaxaca.