The prohibition: It is forbidden to stare at the beauty of an Erva [which includes all girls who have reached the age of menstruation]. One who stares at even the small finger of a woman with intent to benefit from it, is considered to have stared at her most intimate area. [This applies whether the woman is a Jew or gentile.] It is forbidden to stare at the beauty of even a non-Erva unless one is doing so for marriage purposes, as explained in C. In all cases, it is forbidden to stare at a woman in a promiscuous fashion even if she is not an Erva and one may desire to marry her.
Biblical or Rabbinical: Some Poskim rule it is a Biblical prohibition to look at any part of an Erva for pleasure. Other Poskim rule it is only a Rabbinical prohibition to do so. According to all opinions, it is only a Rabbinical prohibition to look at a single girl who is not an Erva [i.e. not a Niddah]. According to all opinions, it is a Biblical prohibition to entertain forbidden thoughts of any woman, even a single woman that is not an Erva.
Punishment and reward:
The gravity of the sin: Looking at woman is one of the sins that is most difficult to do Teshuva for being that people think there is no true sin involved as they have not committed any act. In truth, it is a grave sin, and is the opening of all matters of immorality. Chazal state that whoever looks at Arayos his strength is lost. This means that one who stares at a married women corrupts and weakens his G-dly soul and good inclination. One who looks at woman will not be saved from judgment in Gihennom even if he has Torah and Maasim Toivim. In Kabala it is explained that one who is accustomed to look at women and does not guard his eyes will be reincarnated into a Reiah.
The reward for holding back: Chazal state that whoever controls his eyes from looking at Arayos merits to receive the countenance of the Shechina. This means that one who abstains from partaking in the pleasure of looking at a married woman merits to see and experience the pleasantness of Hashem.
Be as light as an eagle: One is to be as light as an eagle. This refers to one’s sight, that one should close his eyes to avoid viewing evil. Just like the eagle changes his glance very swiftly when he flies across the heavens, so too one should swiftly close one’s eyes when seeing evil. Seeing evil is the beginning of sin, as the images which the eyes see is transformed into a desire of the heart which persuades one to sin.
Sparks of Chassidus
The Rebbe Rashab states the following message regarding the importance of Shemiras Eiynayim [guarding one’s eyes from seeing forbidden sights]:
Forbidden sights and glances is the cause of all matters of evil, as the Sages state that the eyes and heart are the two instigators of sin, as one who looks in the end will come to sin. It is known that the main feeling of pleasure is found in the sense of sight; hence explaining why a blind person does not have pleasure in any matter [and is considered like dead]. It is for this reason that sight arouses pleasure in oneself. This is visibly witnessed even amongst those that are very distanced from evil, that they are able to be aroused towards evil through sight. It captures them into a bucket of filth and leads them to the depths of immorality. Therefore every person who cares for his soul and desires to prevent it from falling into the pits of impurity is to control his sense of sight. Even if this matter is very difficult for him to control, he should arouse within himself a mighty and powerful will to overcome it, having the knowledge of its importance. If one does not control his sight all of his service is considered like naught as he will be unable to affect any true inner feeling of service of G-d, and on the contrary he will descend in a downwards spiral from his level of service. Therefore one must express within himself the strongest will power to overcome it.
Ignore the scoffers: Even if doing so will make a mockery of himself in front of others he should pay them no attention. Is it not that with regards to material matters which relate to ones very life that one ignores the public opinion and acts in accordance to the way he is required to save himself? Certainly then one should do so with regards to spiritual matters which relate to the eternal life of the soul.
The battle is only in the initial moments: The difficulty in controlling ones sight is in truth only on the onset of one’s efforts to take control of his past nature, however with time he will become accustomed to it and its difficulty will cease.
Even a cold glance remains permanent within the soul: Even if one does not feel any conscious feeling or emotion upon glancing at forbidden sights, he should be aware that in truth the sight that he views penetrates to the essence of pleasure that is found in his soul and will eventually, Heaven forefend, erupt itself into his conscious mind. In fact it is for this reason that he is glancing at the forbidden image despite him having no conscious pleasure from doing so, as the essence of his animal soul is so involved within the sight that it is unable at the current moment to reveal the pleasure to the conscious mind.
In summary: “It is an obligation on every individual to overpower himself and control his sight and through doing so he will save his soul from evil and his service of Hashem will be appeasing and effect salvation within his soul”!
The eyes are considered blind for the moment of the forbidden glance:
“In answer to your question regarding the matter of Shemiras Eiynayim, it is understood that there is no place to write this in letters besides for the fact that there is no need to do so being that the matter is explained in the teachings of Chassidus. In general the importance of it can be understood from the saying of the Sages that a blind man is considered like he is dead and hence staring at a forbidden matter which is contrary to the purpose of existence of the eyes is considered a temporary blindness, Heaven save us, and even worse so. It is not necessary to lengthen on this obvious matter.”
Crying is Tikkun for sins of sight:
It states in Sifrei Mussar that crying is a Tikkun for the forbidden pleasures which one allowed his eyes to partake in. When one now uses his eyes to express complete pain it fixes this sin.
 M”B 75/7
 Michaber E.H. 21/1; Braisa in Avos Derebbe Nasan; See Eiruvin 18b “One who gives money to a woman in order to look at her, is not saved from judgment of Gihennom”; Nedarim 20a “One who stares at women in the end will come to sin.”; Avoda Zara 20a “One may not stare at a beautiful woman even if she is single, and may not stare at a married woman even if ugly”; M”B 75/7
 Michaber E.H. 21/1; Rav Sheshes in Shabbos 64b
 Michaber E.H. 21/3; Avoda Zara 20a “One may not stare at a beautiful woman even if she is single, and may not stare at a married woman even if ugly”
The reason: On this the verse states “Bris Karisi Laeiynayim Umah Esbonan Al Besula.” [Michaber ibid]
 Michaber Even Haezer 21/1; Tur 21/1 as explained in Perisha 21/2; Riy, brought in Beis Shmuel 21/2; Orchos Chaim 2/13; Rabbeinu Yona Igesres Hateshuvah 19/20, brought in Beis Yosef 21; M”B 75/7
 The reason: It is Biblically prohibited to look at an Erva due to the verse “Lo Sasuru Acharei Levachem Veachareiy Einechem”. [Riy, brought in Beis Shmuel 21/2; Beis Yosef ibid in name of Rabbeinu Yona and Orchos Chaim; M”B 75/7]
 Rambam, brought in Beis Shmuel 21/2
 Beis Shmuel 21/2; Beis Yosef ibid in name of Rabbeinu Yona and Orchos Chaim
The reason: This is learned from Divrei Kabala [i.e. prophets] in the verse in Iyov 31/1 “Bris Karisi Lieiynaiy Umah Esbonan Al Besula” [Beis Yosef ibid in name of Rabbeinu Yona and Orchos Chaim]
 Beis Shmuel 21/2; Beis Yosef ibid in name of Rabbeinu Yona and Orchos Chaim “It is more severe to have forbidden thoughts of a Penuya than to even touch her, as through thinking one transgresses a Biblical negative command, as the verse states “Vinishmartem Mikol Davar Ra.”
 Rambam Teshuva 4/4
 Sanhedrin 92a
 See Rashi ibid for two explanations there
 Beis Yosef ibid in name of Rabbeinu Yona and Orchos Chaim
 Eiruvin 18b; M”B 75/7
 Shaar Hagilgulim of Arizal, brought in Sefer Chareidim 33
 Midrash Raba Acharei Mos 23/13
 This is learned from the verse [Yeshayahu 33/15] “Otzem Eiynav Meros Bera” and it then says in verse 33/17 “Melech Beyafyo Sechezena Eiynecha.” [Midrash ibid]
 Beis Yosef ibid in name of Rabbeinu Yona and Orchos Chaim
 Tur and Beis Yosef O.C. 1
Opinion of Admur: The above explanations are taken from the Tur, and are not mentioned by Admur. Possibly the reason for this is because Admur follows the order of the Michaber and Rama, which also did not explain the above. [Ketzos Hashulchan 1 footnote 3]
Opinion of Bartenura: Rabbeinu Ovadia Bartenura comments on this statement that it refers to that one should review his studies and this will cause him to effortlessly be expert in the topic.
 So is implied from commentary of Beis Yosef on the Tur. As for why the term “light” was used, seemingly this is saying that just like the eagle due to his lightness is able to fly quickly past sightings, so too one’s eyes are to be light and swiftly turn away from viewing evil.
 Kuntrus Haavoda p. 11: Below is a free translation of selected parts of the Mamar. For the full version of the words of the Rebbe Rashab on this topic refer to the original Kuntrus.
 A proof for this can be found in the Mishneh [Zavim 2/2] that one can see an emission from merely seeing an immoral sight even if he did not think about it. From this we learn the great care one must take to control his sense of sight. [Rebbe in his glosses on Taharas Yom Tov Vol. 9 p. 19; printed in Igros Kodesh]
 Igros Kodesh 19 p. 344 [letter 7342]
 Derech Chaim 20b