פירוש על קידושין 3:2

To the deceased husband's widow [yavam]

"The Yevamah is acquired..." Problem: [The Mishnah] did not teach this in the same form as the list above—let it teach "She is acquired in 1 way and acquires herself in 2 ways"! And let it be like the earlier list [which comes] to exclude as we say in the Gemara: let this list also come to exclude; the beginning is to exclude money and contract, as [the Talmud] explains later, "Sex completes [the acquisition], but money and contract do not complete", and the list at the end to exclude also that we can't derive through ק״ו from the woman, as we say in the Gemara that we explain: "His shoe—yes; anything else—no"! Solution: For one item, one does not repeat the list form. [Problem:] But even though regarding etrog we teach "It is equal to a tree in 3 ways and to a vegetable in 1 way", [solution:] there it comes to explain the things in which it differs [as opposed to here, which are just independent lists]. [Solution on a new topic:] Since [the Mishnah] did not teach the list form for the yevamah, it was not worried to repeat the list form for Hebrew or Canaanite slave. Alternatively: because it is not needed to teach about them in list form to exclude anything [since there is nothing to exclude].

To be his wife for all matters, for if he came to divorce her afterward, she does not need from him halitza rather a bill of divorce [get]. But money or a document do not have the effect of betrothing her from the Torah, rather from the Sages as they instituted an order, as they say in Yevamot. But (money or a document) do not finalize the betrothal to allow him to inherit from her or become impure for her and do not exempt from halitza, rather they make her forbidden to the other brothers.

A woman becomes halakhically separated from her husband either by divorce or by death. Without one of the two, any relations that she has with another man will be considered adultery.

The dead husband’s brother-in-law “acquires” his brother’s widow through sexual intercourse. The yevamah is not acquired by money, as a woman would be in cases of normal betrothal.
The yevamah is free to marry another man if she performs halitzah with the yavam. Alternatively, if the yavam dies (in a situation where there is only one yavam) she also may marry anyone she so chooses. Note that once she is married she is considered a normal wife, and she “acquires” herself through the death of her husband or through divorce.