Before us is Mishnah 5 in the sixth chapter of Maseches Nedarim, which deals with the laws of one who takes a vow forbidding himself milk, kum, or cheese. The question underlying the Mishnah is what is included in the wording a person used in his vow, and as we shall see, the answer depends on the way people in a given place speak.
"HaNoder min hechalav - mutar bakum":
'Kum' is what we call whey - the watery substance that remains after milk has curdled into cheese. According to the Tanna Kamma, one who vowed not to benefit from milk or to eat it is permitted in kum, since kum is not included in the category of milk.
"Rabbi Yosei oser":
According to Rabbi Yosei, one who vowed not to eat milk is forbidden in kum as well. The Gemara explains that their dispute stems from differences in local speech:
In Rabbi Yosei's locale: people would regularly call kum "the kum of the milk," and for that reason it is regarded as part of the milk and is included in the prohibition of the vow.
In the Tanna Kamma's locale: they called milk "milk" and kum "kum" - two separate names, and therefore kum is not included in a vow against milk.
"HaNoder min hakum - mutar bechalav":
Here all agree. True, one might have thought that kum is considered a type of milk, but the reverse is not so: milk is not included in a vow against kum.
The words of Abba Shaul:
"HaNoder min hagevinah - asur bah bein melucha bein tefeilah" - even though the one vowing used the wording "the cheese" with the definite article, we do not say that he intended a particular type of cheese; rather, his vow includes every cheese, whether salted or unsalted.
There were two lines of reasoning one might have raised, according to which he intended specifically salted cheese:
Because of the definite article, which seemingly points to a particular, defined type.
Because most cheeses were salted, and therefore even saying "cheese" plainly - without the definite article - is directed at them.
The Mishnah teaches us that this is not so: the wording "the cheese," despite the definite article and despite the fact that most cheeses are salted, includes salted and unsalted cheeses alike.
In summary: in this Mishnah we learned that one who vows against milk is permitted in kum according to the Tanna Kamma and forbidden according to Rabbi Yosei, and it all depends on the local manner of speech; one who vows against kum is permitted in milk according to all opinions; and according to Abba Shaul, one who vows against cheese is forbidden in every cheese, whether salted or unsalted.