The first difference: qualities
Semen in the case of men is a thick, white liquid, and in the case of women it is thin and yellow. (Translators note: the word maniy is used to refer to the fluid secreted by either male or female at the time of orgasm).
The source for these descriptions of maniy come from the hadeeth of Umm Sulaym (may Allaah be pleased with her), who asked the Prophet (peace and blessings of Allaah be upon him) about a woman who sees a dream like a man sees (i.e., an erotic dream). The Messenger of Allaah (peace and blessings of Allaah be upon him) said: If a woman sees that, she should do ghusl. Umm Sulaym said: I felt shy when I heard that. She asked, Is that possible? The Prophet (peace and blessings of Allaah be upon him) said: How else would the child resemble his father or mother? The water of the man is thick and white, and the water of the woman is thin and yellow. Whichever of the two prevails or comes first decides which parent the child will resemble.(Agreed upon. Saheeh Muslim, 469).
In his commentary on Saheeh Muslim (3/222), Imaam al-Nawawi said, concerning the words of the Prophet (peace and blessings of Allaah be upon him) The water of the man is thick and white, and the water of the woman is thin and yellow: This is an important criterion in describing maniy. This description applies when a person is healthy, and in most cases. The scholars said: when a man is healthy, his semen is white and thick, and gushes out in spurts accompanied by feelings of pleasure. After it has come out, it is followed immediately by a feeling of being drained and exhausted. Its smell is like that of palm tree pollen, which in turn resembles the smell of dough (the colour of semen may change for a number of reasons, including the following): If a person is sick his semen may become thin and yellow, or his testicles may become slack or soft, with the result that the emission of semen is not accompanied by any feelings of pleasure. If a man engages in intercourse too frequently, his semen may become red and resemble the juice that comes from meat, or may even contain blood [ameet???]. There are three distinguishing features of semen, the presence of any of which is sufficient for it to be classified as semen: firstly, its emission is accompanied by feelings of pleasure and is immediately followed by exhaustion; secondly, it smells like palm tree pollen, as stated above; and thirdly, it gushes out in spurts. Each one of these three characteristics is sufficient for it to be defined as semen; it is not necessary for all three features to be present. If none of them are present, the substance cannot be described as semen and in all likelihood it is not. All of the above applies to the maniy of men; in the case of women, their maniy is thin and yellow, although it could become white if the woman is strong. It has two distinguishing characteristics, either of which is sufficient to class the substance as maniy: firstly, it smells like the maniy of men, and secondly, its emission is accompanied by feelings of pleasure and is immediately followed by exhaustion.
Madhiy, on the other hand, is a sticky white fluid that is emitted when a person is thinking of or desiring intercourse. Its emission is not accompanied by feelings of pleasure, it does not gush out in spurts, and it is not followed by exhaustion. This may happen to men and women alike, although it is more common among women. This is the comment of Imaam al-Nawawi in his commentary on Muslim(Sharh Muslim, 3/213).
The second difference: what should be done after emission of either type of fluid
When maniy is emitted, a person must perform ghusl to clean himself from sexual impurity (janaabah), whether this happened during intercourse or otherwise, whether he was awake or it happened because of an erotic dream. Madhiy, on the other hand, means that a person only needs to do wudoo. The evidence for this is the report of Ali ibn Abi Taalib (may Allaah be pleased with him) who said: I was a man who used to experience a lot of urethral discharge, so I told Miqdaad to ask the Messenger of Allaah (peace and blessings of Allaah be upon him) about it. So he asked him, and he said: It requires wudoo.(Agreed upon; this version was narrated by al-Bukhaari). Ibn Qudaamah said in al-Mughni (1/168), Ibn al-Mundhir said: The scholars agree that passing stools or urine, the emission of madhiy or passing wind are all things that invalidate tahaarah (purity) [i.e. they mean that wudoo must be renewed].
The third difference: whether they are taahir (pure) or naajis (impure)
Maniy is taahir according to the most correct opinion among the scholars. The evidence for this is the report of Aaishah (may Allaah be pleased with her) who said: The Messenger of Allaah (peace and blessings of Allaah be upon him) used to wash the maniy (from his clothes) then go out to pray wearing the same clothes, and I could see the spot where he had washed it. (Agreed upon). According to a report narrated by Muslim: I used to rub it (semen) thoroughly from the garment of the Messenger of Allaah (peace and blessings of Allaah be upon him), then he would pray in it. According to another version: I would scratch it from his garment with my nail when it had become dry. It is proven that the Prophet (peace and blessings of Allaah be upon him) used to leave it without washing it when it was wet; it was sufficient just to wipe it with ood (a kind of aromatic wood) or something similar, as was reported by Imaam Ahmad in his Musnad (6/243) from Aaishah (may Allaah be pleased with her), who said: The Messenger of Allaah (peace and blessings of Allaah be upon him) used to remove maniy from his garment with a twig of adhkhar (a kind of tree), then pray in that garment, and he would rub it from his garment once it was dry, and would pray in the same garment. Ibn Khuzaymah reported it in his Saheeh, and Shaykh al-Albaani classed it as hasan in al-Irwa (1/197).
Madhiy (urethral discharge) is naajis (impure), because of the hadeeth of Ali quoted above, in some versions of which it is reported that the Prophet (peace and blessings of Allaah be upon him) commanded him to wash his penis and testicles and do wudoo, as was reported by Abu Awaanah in al-Mustakhraj. Ibn Hajar said in al-Talkhees: There is nothing at fault in this isnaad. So madhiy is naajis and the penis and testicles must be washed, as it nullifies tahaarah.
Ruling concerning clothing on which there is semen or urethral discharge
As maniy is deemed to be taahir, if it gets on a persons clothes it does not make them impure, and if he prays in those clothes there is nothing wrong with that. Ibn Qudaamah said in al-Mughni (1/763): Even though we say it is taahir, it is better to rub it, but if one prays (in those clothes) without rubbing it, this is acceptable.
In the case of madhiy, it is sufficient to sprinkle water on it, because of the difficulty of doing otherwise. The evidence for this is the hadeeth narrated by Abu Dawud in his Sunan from Sahl ibn Haneef, who said: I used to experience a lot of urethral discharge, and I used to do ghusl a lot. I asked the Messenger of Allaah (peace and blessings of Allaah be upon him) about it and he said: It is sufficient for you to do wudoo (to purify yourself) from that. I said, O Messenger of Allaah, what about when it gets on my clothes? He said: It is sufficient for you to take a handful of water and sprinkle it on your clothes wherever you think (the madhiy) is.(Reported by al-Tirmidhi. He said: This is a saheeh hasan hadeeth, and the only other hadeeth about madhiy that we know is the hadeeth of Muhammad ibn Ishaaq).
The author of Tuhfat al-Ahwadhi (1/373) said: This may be taken to prove that if madhiy gets on one's clothes, it is sufficient to sprinkle water on it; one does not have to wash it.
And Allaah knows best.