Yevamos, Chapter Two, Mishnah 4. In the previous Mishnah we learned that a yevamah who falls under "issur mitzvah" or "issur kedushah" - as opposed to an ervah - both she and her co-wife require chalitzah, but they cannot be taken in yibum because of the prohibition that rests upon them. Our Mishnah comes to define what issur mitzvah is and what issur kedushah is.
Issur Mitzvah:
As we mentioned briefly in the previous Mishnah, there are relatives who are forbidden not by Torah law but by rabbinic law, such as:
His mother's mother.
His father's mother.
His son's daughter-in-law.
His daughter's daughter-in-law.
If a man's brother was married to a woman who falls into this category, and she comes before him for yibum, she requires chalitzah, while yibum is forbidden.
Issur Kedushah:
The Mishnah lists several cases:
"Almanah l'Kohen Gadol" - a widow married to a Kohen Gadol, who is forbidden to him.
"Gerushah va'chalutzah l'kohen hedyot" - a divorcee or a chalutzah married to an ordinary kohen. According to most opinions, the prohibition of a chalutzah to an ordinary kohen is rabbinic.
"Mamzeres u'nesinah l'Yisrael" - a mamzeres is the offspring of a relationship punishable by kares; the Nesinim were a group of converts to Judaism whom it was forbidden to marry.
"Bas Yisrael l'nasin u'l'mamzer" - the reverse relationship: a Jewish woman who married a nasin or a mamzer.
All of these are considered issur kedushah, and they are forbidden by Torah law. In any case, yibum is impossible because of the prohibition, but there is an obligation to perform chalitzah.
According to the Gemara, these prohibitions are called "issurei kedushah" because the term "kedushah" appears in the passage in which the kohen is told whom he may not marry. From there the name extends to the prohibition of a nasin and a mamzer to a Jewish woman as well, since the matter touches on the sanctity of lineage and the sanctity of the family line, and it too is therefore called issur kedushah.
In summary: This Mishnah defined the two categories of prohibition mentioned in the previous Mishnah. Issur mitzvah - relatives forbidden by rabbinic law, such as his mother's mother, his father's mother, his son's daughter-in-law and his daughter's daughter-in-law. Issur kedushah - a widow to a Kohen Gadol, a divorcee and a chalutzah to an ordinary kohen, a mamzeres and a nesinah to a Jewish man, and a Jewish woman to a nasin or a mamzer. In both categories the law is the same: there is no yibum because of the prohibition, but there is an obligation of chalitzah.