What is meant by an angel taking on the form of a human is that an angel is visually perceived by humans as having taken on the physical shape of a person. This does not mean that the essence of the angel has changed or that it has become something else (other than its original self); but a person viewing the angel will see it in the figure of a human. In brief, the appearance of an angel as a human is as a result of the human’s perception of the angel as a person, not as a result of a change in the essence or nature of the angel itself. Therefore, an angel that takes on the form of a human is in reality the same being that it was, and does not possess human traits like lust or the need to consume food.
Prior to explaining the main topic at hand, it is necessary to explain two terms:
One: The word “Malaaikah” (angels) is the plural of “Malak”, which are creatures created by Allah (swt) who serve as an intermediary between God and the material world.
Two: The word “Tammathul” is derived from the root word “Mithl”, and is used to describe something that has taken on another form apart from its original.
The essence of angels in religious teachings has been described as follows:
In the Holy Quran, there exists a host of topics regarding angels and their actions. However, there is no explicit mention of their nature or makeup.
In the book Bihar, it has been narrated from Imam Sadiq (as) that: “Allah Almighty has created angels from light.”
It has been narrated in Tafsire Qummi that Imam Sadiq (as) said: “Angels do not consume water or food and do not marry. The breeze of the empyrean (Arsh) maintains their existence. For Allah (swt), there are angels that until the Day of Judgment are continually in ruku (bowing), and others that until the Day of Judgment are continually in sajdah (prostration).”
Imam Abujafar (as) has said: “Allah (swt) created Israfil, Jibra’il, and Mika’il from one tasbih (glorification), and provided them with ears, eyes, intelligence, and swift understanding.”
In regards to the essence and reality behind angels, various opinions have been expressed:
1) Some say: angels inhabit space and are material beings composed of material similar to air. They are believed to appear in a host of different forms and reside in the sky.
2) Clans of idol worshippers used to say: the stars within the sky are angels of welfare and misery. They believed the stars to be alive and capable of rational thought. The stars of welfare were thought to be the angels of mercy, while the stars of misery were thought to be the angels of torment and punishment.
3) From a philosophical point of view, it is believed that: an angel is an immaterial being that does not inhabit any form of space, and in essence differs greatly from human beings. They are also believed to be more powerful and knowledgeable than humans.
In hadiths, the word tammathul (taking the form of) is seen quite often. However, in the Holy Quran, it is not mentioned in any surahs besides surah Maryam. The verses following the verse that speaks of the event of Jibra’il revealing upon Lady Maryam serve as the best proof that when Jibra’il, while displaying himself in the form of a human, he was in reality still an angel. As opposed to actually becoming a human, Jibra’il only took the shape of a human, thus causing Maryam to see him in that form.
Therefore, the manifestation of Jibra’il in human form was as a result of Maryam’s perception and observation of him, not as a result of an actual change, thus he was still an angel and had the form of one for anyone other than her (and probably wouldn’t even be seen by anyone else).
Therefore, the meaning of the word tammathul used here, is considering the literal definition of the word. When it is said: «تمثل شىء لشىء فى صورة کذا», what is meant is that something appears for someone in the shape of another form (other than its own); meaning that one perceives the angel in the form of another being, as opposed to the angel actually changing into something else. Therefore, the tammathul of an angel in the form of a human is only in regards to the perception of the viewer.
Several faults have been presented concerning this statement, of which one will be mentioned and given a response to:
The basis of one fallacy states that from what we have been told from hadiths regarding Jibra’il, he is an enormous being. Thus, if he were to take the form of a human being, he would have to either become smaller in size or a part of him would have to be lessened. In these cases, Jibra’il would either be fundamentally changed or he would have to fit into a smaller form while simultaneously maintaining his own form which is logically unfeasible.
The response that has been stated is that if we believe Jibra’il to be a physical being, it is possible for him to have two different forms of physical makeup. The first could be his primary physical form, which is the same as normal human beings. The second could be a form in which excess physical parts are added to the main one. This would allow him to present himself in his primary human form anytime he likes. If we believe Jibra’il to be immaterial spiritual being, then it is reasonable to assume that he can take on forms large or small, like those mentioned in narrations.
However, the mentioned answer is flawed since its first section presupposes that tammathul is the result of an actual physical transformation. This would imply that the one undergoing tammathul, would be fundamentally changed from its original state and transformed into another. Conversely, as previously stated, what we mean when we say tammathul is not a process in which the essence of the being in mind is at all changed, but rather preserved completely (the only thing changing is the entity’s appearance for a certain person/group). The prospect assumed in the previous answer would be essentially different from what we have mentioned.
When taking into consideration the context of the verse of the Quran in which the story of Maryam is mentioned, it appears that Jibra’il when undergoing tammathul was not changed from his original form of an angel and was not transformed into a human being. Rather, it was Maryam’s perception of him as a human, not an actual physical change that made it seem as if he had transformed. Other cases of tammathul can also be found in history. For example: the noble angels descend upon Prophet Ibrahim (as) and gave him glad tidings before the birth of his son Ishaq (as), their revelation upon Lut (as) in the form of humans, and the manifestation of Iblis at the battle of Badr in the appearance of Suraqah bin Malik, when in reality Suraq bin Malik was in Makkah on that day.
Examples of this kind of tammathul can be found extensively within narrations. One instance of this is regarding Iblis in the story of «دار الندوة». In the event that took place, the mushrikin were plotting to kill the Messenger of Allah (pbuh). They were consulting with one another, when Iblis appeared in the form of an old man and suggested a new method and plan that was commended by those present. On another occasion, Iblis appeared in the presence of Yahya (as) on the day of Aqabah with the appearance of Munnabbahh bin Hajjaj. For Amir al-Muminin (as), this world presented itself to him in the form of a youthful and attractive woman. For those passing away, wealth, progeny and deeds are presented through means of tammathul to that person. A person in their grave or on the Day of Judgment will see their deeds and actions manifested to them through tammathul. What is seen in dreams is also in line with this same nature of tammathul. For example: the appearance of an enemy in the form of a dog, snake, or scorpion, the appearance of a spouse in the form of shoes, the appearance of development and increase in status in the form of a horse, and the appearance of honor in the form of a crown. These examples and others of the same nature are all examples of tammathul. 
With the explanation that has been given, it becomes clear that the reality and essence of an angel is never changed, nor does it ever physically transform into a human being. In reality, only tammathul takes place, allowing the angel to take the shape of another entity. Therefore, an angel that appears in the form of a human is not comprised of human qualities such as lust, desire, or the need to consume food and water. Moreover, angels live by standards and rules unique to their own kind, which is different than that of the physical realm of humans.
One thing that should be mentioned is that the possible basis of this question may be a result of incorrect views or superstitious and false stories spread by groups within the Jewish faith. For example, in the story of Harut and Marut (the names of two angels) which is mentioned in the Quran, we can witness the mischievous and deceitful roles these groups have played. Regarding the angels mentioned in the Quran, they have fabricated false stories concerning excessive lust, desire, and sinning on behalf of these two angels. They spread these false claims amongst people and have even mentioned some within their own books.
For further information and explanation, it is necessary to refer to the Shia books of tafsir (commentary).
 Question 1853 (website: 1922).
 Ragheb, Mufradat, vol.1, pg. 758.
 Question 1853 (website: 1922).
 Mohammad Baqer Kamare’i, translation of Kitabul-Sama’ wal-Alam, Biharul-Anwar, vol. 3, pg. 173.
 Maryam:17 “فَأَرْسَلْنا إِلَیْها رُوحَنا فَتَمَثَّلَ لَها بَشَراً سَوِیًّا”.
 Muhammad Husien Tabatabai, Al-Mizan (translation f Musavi), vol. 14, pp. 47-48.
 Muhammad Husein Tabatabai, Al-Mizan (translation of Musavi), vol. 14, pg. 49. In order to learn about Iblis appearing as Suraqah bin Malek, refer to Al-Mizan, under verse 48 of surah Anfal; also, for further information on the tamathul of angels in the forms of humans, see Al-Mizan, under verse 17 of surah Maryam.
 Mohammad Husein Tabatabai, Al-Mizan (translation of Musavi), vol. 14, pg. 49.
 The tafsir of Al-Mizan has presented some valuable information on this topic under verse 102 of surah Baqarah.