The Torah exhorts man to “keep his word” and identifies two types of verbal utterances which must be followed: a neder and a shevuah (Numbers 30:3). The word neder is commonly translated as a “promise”, while shevuah is commonly translated as an “oath”. This rendering implies that the two words are basically synonymous, with a shevuah being a more serious type of neder. However, the Talmud (Nedarim 2b) understands that neder and shevuah are two different types of verbal expressions, and each must be dealt with on its own terms. In fact, the Mishnah has two separate tractates in two separate Mishnaic Orders which deal with these laws: Nedarim is in the Order of Nashim and Shevuot is in the Order of Nezikin.
A shevuah is an oral statement that generally invokes the name of G-d, and focuses on the person who makes the statement. These statements may take positive or negative forms, and may refer to either the past or the future. For example, in a shevuah a man says, “I swear I will do such-and-such”, “I swear I will not do such-and-such”. Similarly: “I swear I did do such-and-such”, or “I swear I did not do such-and-such”. Additionally, in the course of civil litigation a Jewish court may impose two more types of shevuot: One type of shevuah is imposed on a litigant in order to substantiate the veracity of his argument(s), and the other type of shevuah is imposed on potential witnesses who were subpoenaed by the court, but claim they have nothing to testify. All of these shevuot must be fulfilled or must refer to something that is already true.
There is also an illegal type of shevuah outlawed by the third of the Ten Commandments. That commandment forbids one from taking G-d’s name in vain, and refers to four types of vain oaths: Swearing about something which is patently false (“I swear that this marble column is gold”), swearing about something which is blatantly obvious (“I swear that the heavens are the heavens” and other examples of reflexive redundancies), swearing to violate the law of the Torah (“I swear that I will not wear tefillin”), and swearing to do something impossible (“I swear I will refrain from sleeping for three full days”).
A neder is a verbal declaration which creates a halachic status shift, and focuses on an object instead of on the person making the declaration. For example, a neder can be used to create a prohibition from deriving benefit from another thing or person, or from eating a particular foodstuff. The mechanics of a neder essentially renders the object of one’s verbal declaration into quasi-consecrated property, thereby marking it as forbidden. In other words, a neder mimics the procedure for making an object into the holy property of the Temple. This type of neder is called a konam(a purposefully bowdlerized form of the word korban — sacrifice). Case in point: If one makes a neder that he will not smoke, he has essentially said, “I promise that benefits derived from cigarettes shall be forbidden to me, just like the use of a consecrated animal is forbidden to me”. Indeed, when one promises to donate a sacrifice or other component to the Holy Temple, that pledge is also called a neder.
Another type of neder is called a nedava, which is essentially one statement that includes the promise to offer a sacrifice (sometimes only implicitly), plus the designation of a specific animal as said sacrifice. E.g., if one points to his ram and says, “This ram shall be a burnt-offering”, he is essentially obligating himself to bring a burnt-offering, and is also designating that ram as the consecrated item.
In short, both a neder and a shevuah refer to oral pronouncements that have legal standing in the Torah, but their points of foci are on different aspects. The neder focuses on a specific object, rendering forbidden that object or the benefits which can be derived from it. The shevuah focuses on the party pronouncing the oral declaration, and obligates him to do or not do a specific action. All in all, man’s power to create these types of legal realities by way of words shows the influence and importance granted to the concept of speech. Indeed, King Solomon said, “Death and life are in the hands of the tongue” (Proverbs 18:21). Because of the gravity of such matters, it is customary to render null and void all nedarim and shevuot on the eve of Rosh Hashana and Yom Kippur in order to save people from potentially violating their own verbal utterances.