'Beher' in Urdu Poetry is the 'meter' of a Sher. It can be considered as the length of the Sher. Both the lines in the Sher, the couplet, *MUST* be of same 'Beher'. And all the Sher's in one Ghazal *MUST* be of the same 'Beher'. There are 19 (!!!!) kinds of 'Beher'. However generally 'Beher' is categorized in 3 classes:
Short, medium, long
depending upon the length of the misra (first line of the Sher)
For a Ghazal, all the Shers in the Ghazals should be of the same of Beher. The example of this is this Ghazal of Faiz Ahmed Faiz. Notice the length and meter of the Ashaars in this Ghazal, it is same throughout.
shaiKh saahab se rasm-o-raah na kii
shukr hai zindagii tabaah na kii
tujh ko dekhaa to ser-e-chashm hue
tujh ko chaahaa to aur chaah na kii
tere dast-e-sitam kaa ajz nahii.n
dil hii kaafir thaa jis ne aah na kii
the shab-e-hijr kaam aur bahut
ham ne fikr-e-dil-e-tabaah na kii
kaun qaatil bachaa hai shahar me.n 'Faiz'
jis se yaaro.n ne rasm-o-raah na kii
