Massekhet Temurah deals with all aspects of exchange of sacrifices, of which the central aspect is the prohibition against attempting to remove the sanctity of a sacrificial animal by exchanging it with an ordinary animal that will be sacrificed in its place. All of these laws are derived from two short passages in Sefer Vayikra (27:10), which state that someone who tries to exchange a sanctified animal will result in both animals becoming sanctified.
There are two unique aspects to the laws of temurah:
1. Although it is forbidden to perform the act of exchange, and, indeed, someone who does so will be liable to receive lashes, nevertheless the sanctity takes effect and is legally binding. The idea that a forbidden act creates a reality that is recognized by Jewish law applies in other cases, as well, as is discussed in the Gemara.
2. By means of the mechanism of temurah, animals that ordinarily could not become sanctified as sacrifices (e.g. they suffer from a permanent blemish) will acquire that level of holiness. The Gemara discusses what, then, can be done with such animals.
Related to this last issue, the Gemara in this tractate also discusses how to deal with the embryo (or newborn) of a sanctified animal. If a female animal that has been sanctified to be brought on the altar becomes pregnant, what is the status of the offspring? In some cases the offspring can be brought as a sacrifice similar to that of its mother. In other cases, for example if the offspring is male, the animal cannot be brought as the same korban as its mother. What is to be done with such an animal?
Furthermore, can the owner of a pregnant animal declare that the mother is sanctified as one type of sacrifice while the embryo should be brought as a different one?
Since the creation of a sanctified animal by means of temurah is forbidden, the discussion of this phenomenon leads to a discussion of parallel cases – sacrifices that do not have blemishes but nevertheless cannot be brought. Examples of these include animals that cannot be sacrificed because of something that was done to them (e.g., if they were used as the object of idol worship) or the cases of etnan zonah u’mehir kelev – an animal that was used to pay for prostitution or exchanged for a dog (see 23:19).