We begin with the first Mishnah in the ninth chapter of Maseches Yevamos, which opens with the statement: "Yesh mutaros le'va'aleihen va'asuros li'yvameihen" - there are women who are permitted to marry their husbands, but if the husband dies childless and the woman comes before the yavam for yibum, she is not permitted to the yavam - the husband's brother - even though she was permitted to her late husband.
The four categories listed by the Tanna:
"Mutaros le'va'aleihen va'asuros li'yvameihen" - women who are permitted to their husbands and, under certain circumstances, forbidden to the yavam.
"Mutaros li'yvameihen va'asuros le'va'aleihen" - women whose marriage to their husbands was a forbidden marriage, while it is the brother to whom they are permitted to marry.
"Mutaros la'eilu vela'eilu" - women who are permitted both to their husbands and to their yevamim.
"Asuros la'eilu vela'eilu" - women who are forbidden to both brothers, both to the late husband and to the yavam.
"Ve'eilu mutaros le'va'aleihen va'asuros li'yvameihen":
"Kohen hedyot shenasa es ha'almanah veyesh lo ach kohen gadol" - a widow is permitted to an ordinary kohen, while his brother the yavam, who is a Kohen Gadol, is forbidden to marry a widow.
"Chalal shenasa kesheirah veyesh lo ach kasher" - a chalal is one whose father is a kohen and who is the product of a forbidden marriage of a kohen. A woman of valid lineage is permitted to marry him and there is no prohibition whatsoever in this marriage, except that by the very definition of the union she becomes a chalalah. The husband's brother - the Tanna said "kasher," meaning a kohen of valid status - is forbidden to marry a chalalah.
"Yisrael shenasa bas Yisrael veyesh lo ach mamzer" - a Yisrael who married a bas Yisrael, and he has a brother who is a mamzer, born of a forbidden relationship (the same father and different mothers, and therefore one of them is a mamzer). The woman is permitted to her husband and forbidden to her yavam.
"Mamzer shenasa mamzeres veyesh lo ach Yisrael" - a mamzer and a mamzeres are permitted to one another, but his brother, who is not a mamzer but a Yisrael, cannot marry a mamzeres, and so she is forbidden to her yavam.
A precise reading of the Mishnah's wording:
Seemingly the Tanna did not have to choose specifically the case of a widow. Even if the ordinary kohen married a virgin who had never been married, once her husband dies and she comes before the yavam for yibum she is a widow, and the Kohen Gadol is forbidden to her. The mention of a widow, then, is made in passing, on account of the latter part of the Mishnah, where a case of a widow will be needed.
In summary: the Mishnah listed four cases in the first category - an ordinary kohen who married a widow and has a brother who is a Kohen Gadol; a chalal who married a woman of valid lineage and has a brother who is a kohen of valid status; a Yisrael who married a bas Yisrael and has a brother who is a mamzer; and a mamzer who married a mamzeres and has a brother who is a Yisrael. In all of them the woman is permitted to her husband and forbidden to her yavam.
Further on, the Mishnah will detail the remaining categories: those permitted to their yevamim and forbidden to their husbands, those permitted to both, and those forbidden to both.