1. Hijab is not the khimar. The headscarf is the khimar. Everyone, including muslims, calling the khimar the 'hijab' is destroying hijab:
a. Hijaab is a code of modesty comprising a dress code (jilbaab) and a code of behaviour.
b. Jilbaab and behavioural hijab apply to men AND women, whereas the headcovering is not generally considered to be for men (although this is incorrect.)
c. Therefore, calling the khimar 'the hijab' completely destroys the importance of, and even awareness of the existence of male hijab among men and women, non muslim and muslim, creating a patriarchal understanding of the concept.
2. On jilbaab.
a. The female jilbaab is mandated in this verse, (33:59):
“O Prophet! Say to your wives, your daughters, and the women of the believers that: they should let down upon themselves their jalaabib (plural for jilbaab). That is more suitable that they will be known and not be abused. And ever is Allah Forgiving and Merciful.”and then explained in this verse (24:30),
“Say to the believing women that: they should cast down their glances and guard their private parts (from unlawful sexual intercourse - the verse implies a lack of sexually promiscuous behaviour)
and not display their beauty except what is apparent, and they must place their khumur (plural of khimar) over their bosoms...”i.e.
This verse does not necessarily command that women must cover their hair. The verse commands women to pull their khimar, assuming that they're already wearing a headcovering, over their chests - this is because many women of the pre-islamic arab culture used to wear headscarves for environmental reasons while leaving their cleavages exposed.
Some people argue that this means that if women aren't wearing headscarves, they just need to ensure their cleavage is covered - they maintain that the headscarf is mustahaab, or recommended, while not being obligatory.Mainstream contemporary opinions among both sunnis and shias hold the position that the headscarf is obligatory, and that removing the covering of the head from this verse is like saying that it's like wrapping a t-shirt around your stomach and leaving your breasts exposed if someone told you to make sure you were wearing a top that covered your stomach.
b. Male jilbaab is understood mainly from hadiths. Female hijab took precedence at the time of Muhammad (sawas) and so was mentioned in the Quran, because women were being stared at and prejudged as if they were still holding the status of 'property' of men as they had before Islam. Evidence being the second sentence of 33:59, given earlier, and the first sentence of 4:19
"O you who have believed, it is not lawful for you to inherit (be given, perhaps by her father)
women by compulsion."i) Men and women must wear clothes that cover their 'firm' 'awrah (area of modesty) and are recommended to wear clothes that cover their 'soft' 'awrah (area of humbleness). A man's 'firm' 'awrah is defined as the distance between his navel and his knees. His 'soft' awrah is defined as that region + his arms, chest, and legs. It is also recommended, though perhaps unrelated to hijab, to cover his hair.
ii) Women's firm 'awrah is defined as their chest, stomach, hair (majority think this, I don't), and navel to knees. A woman's soft 'awrah is defined as her shoulders, arms, and feet. Some high-ranking scholars have included the face, but in the case of situations where a woman feels that she's being stared at lustfully despite covering both hard and soft 'awrah.
iii) With close family (mahrams, defined as all 'vertical' relations and siblings, not cousins, aunts, nephews etc), only 'hard' 'awrah needs to be followed. For women in front of male family members, this includes everything between the chest and knees, and in front of female family members, just the private parts.the rules are the same for men except they can also uncover their chest. The exceptional circumstances for family rules is when you feel particularly modest or when you're worried there may be incestuous sexual attraction going on somewhere.
iv) When you're around non mahrams of the same sex, you must cover your private parts. This expands to include the soft 'awrah if you're particularly modest, and the hard 'awrah if you feel that you, or one of the people present has homosexual sexual feelings.
3. On behavioural hijab, both men and women are given the same commandments. Don't be sexually promiscuous, e.g. don't uncover your soft 'awrah specifically for the sake of enticing people you're attracted to (or who may be attracted to you) and don't flirt by way of voice or text. Lower your gaze from the apparent attractiveness of men and women who you see and find sexually attractive by what is visible to you of them - and try not to think about people sexually excessively when they're around or not around you.
4. The reasons for this are simple. 'Protection of women' as so many muslims claim is not the reason, because it doesn't take male hijab into account. Hijab protects both men and women, and the only reasons for the differences in 'awrah are due to physiological changes due to the sexual dimorphic nature of the human species. Hijab protects men and women not from rape (though it could in some way,s I guess, but it's not the point) but also unwanted sexual attention as well as negating a phenomenon known in psychology known as 'the halo effect', which you can read about in this paper written by qualified experts, since I'm no expert:
http://deepblue.lib.umich.edu/bitstream/handle/2027.42/92158/TheHaloEffect.pdf