Answer:
Assalaamu alaikum.
Hijab for men
People usually only discuss 'hijab' in the context of women. However, in the
Glorious Qur'an, Allah (swt) first mentions 'hijab' for men before 'hijab' for the
women. The Qur'an mentions in Surah Noor:
"Say to the believing men that they should lower their gaze and guard
their modesty: that will make for greater purity for them: and Allah is
well acquainted with all that they do."
[Al-Qur'an 24:30]
The moment a man looks at a woman and if any brazen or unashamed thought
comes to his mind, he should lower his gaze.
Hijab for women.
The next verse of Surah Noor, says:
" And say to the believing women that they should lower their gaze and
guard their modesty; that they should not display their beauty and
ornaments except what (must ordinarily) appear thereof; that they
should draw veils over their bosoms and not display their beauty
except to their husbands, their fathers, their husbands' fathers, their
sons..."
[Al-Qur'an 24:31]
Six criteria for Hijab.
According to Qur'an and Sunnah there are basically six criteria for observing
hijab:
1. Extent:
The first criterion is the extent of the body that should be covered. This is
different for men and women. The extent of covering obligatory on the male
is to cover the body at least from the navel to the knees. For women, the
extent of covering obligatory is to cover the complete body except the face
and the hands upto the wrist. If they wish to, they can cover even these
parts of the body. Some scholars of Islam insist that the face and the hands
are part of the obligatory extent of 'hijab'.
All the remaining five criteria are the same for men and women.
2. The clothes worn should be loose and should not reveal the figure.
3. The clothes worn should not be transparent such that one can see through
them.
4. The clothes worn should not be so glamorous as to attract the opposite sex.
5. The clothes worn should not resemble that of the opposite sex.
6. The clothes worn should not resemble that of the unbelievers i.e. they
should not wear clothes that are specifically identities or symbols of the
unbelievers' religions.
For Details about halal and haram clothes refer to Sahih Bukhari and Sahih Muslim --> Book of dress.