Nedarim, Chapter 2, Mishnah 1. This mishnah turns on two fundamental principles in the laws of vows:
A davar hanadur as opposed to a davar ha'asur: A vow takes effect when a person attaches his prohibition to something that he himself forbade upon himself by his own declaration, such as a sacrifice, which a person consecrates and thereby forbids to himself. That is a "davar hanadur" - something forbidden by force of a vow and a declaration. If, however, he attached the prohibition to something that the Torah forbade, such as a piece of pork, the vow does not take effect, since this is not something he forbade but something that was forbidden from the outset.
A vow applies to an object and not to an action: A vow forbids an object, whereas an action can be forbidden only by an oath. Therefore one who uses the language of a vow regarding a deed - his vow does not take effect.
"V'eilu mutarin" - vows that do not take effect:
The mishnah lists declarations that cannot create a prohibition, because they were attached to something already forbidden by the Torah:
"Chullin she'ochal lach" - one who says that what he will eat of his friend's food should be considered chullin (ordinary, unconsecrated food). Chullin is not forbidden at all, so there is no vow here.
"K'vasar hachazir" - one who attaches the prohibition to pork, which is something forbidden by the Torah and not something he himself forbade.
"Ka'avodah zarah" - idol worship, which too was forbidden by the Torah.
"K'oros levuvin" - a circular piece of hide created when the heart is removed from an animal offered to an idol. Such a hide has the status of an offering to idolatry, and it is forbidden on its own.
"Kinveilos" - even a kosher animal that died and was not slaughtered properly.
"Kitreifos" - an animal found to have an internal defect that will cause its death, such as an adhesion on the lung.
"Kishkatzim kirmasim" - creeping and crawling creatures that are forbidden to eat.
"K'challas Aharon uch'terumaso" - the challah taken from the dough and the Terumah given to the kohanim. Even though a person separates them himself, their prohibition comes from the Torah and not by force of a vow.
One who says to his wife "Harei at alai k'ima":
This vow too does not take effect, since his mother is forbidden to him by the Torah and not because of a prohibition he created. Nevertheless, so that a person should not accustom himself to making vows against his wife, the Sages ruled "pos'chin lo pesach mimakom acher" - an opening is provided for him from another source.
A 'pesach' (opening) is one of the ways a vow is dissolved: the one who vowed is asked whether he would have made the vow had he known a certain detail at the time. Once he answers that he never would have vowed, this counts as an opening, and on its basis a sage or a beis din dissolves his vow for him. Even though by strict law the vow is void on its own, an alternative reason for dissolution is presented to him - something he did not know at the time of the vow - "shelo yihyu kalei rosh binedarim" - so that people should not treat vows lightly, meaning that he should not come to make light of vows of this kind.
A vow applied to an action:
Here the mishnah returns to the second principle, and lists declarations in which an action is forbidden using the language of a vow:
"Konam she'eini yashein" - that he will not sleep.
"She'eini medabber" - that he will not speak.
"She'eini mehalech" - that he will not walk.
"Ha'omer l'ishto konam she'eini m'shamshech" - that he will not be intimate with her.
All of these are actions, and therefore the vow does not take effect upon them by Torah law. The mishnah does state "harei zeh b'lo yacheil devaro" - he is bound by the prohibition of "he shall not break his word" - but the Gemara explains that this is only rabbinic: by Torah law there is no prohibition, since a vow forbids an object and not an action, and by rabbinic law he is forbidden to sleep, to speak or to walk.
In summary: In this mishnah we learned two rules. First, a vow takes effect only when it is attached to a davar hanadur, such as a sacrifice, and not to something forbidden by the Torah, which is why the vow does not take effect in any of the examples listed in the mishnah. Second, a vow forbids an object and not an action, and therefore "konam she'eini yashein" and the like take effect only by rabbinic law. We also learned that even with a void vow, such as one who says to his wife "harei at alai k'ima", an opening is provided for him from another source, so that people should not treat vows lightly.