Deifnition of Ghazal?

Ghazal is a collection of Sher's which follow the rules of 'Matla', 'Maqta', 'Beher', 'Kaafiyaa' and 'Radif'.

For example:

1. koi ummid bar nahi aati koi surat nazar nahi aati.
2. aage aati thi haale dil par hasi ab kisi baat par nahi aati
3. hum waha hai jaha se hum ko bhi kuch hamari khabar nahi aati
4. kaaba kis mu se jaoge Ghalib sharm tumko magar nahi aati

A 'Sher' is a poem of two lines and every Sher in itself is a poem. So the four stanzas in the above example are independent poems or Sher. Ghazal is a collection of two line poems called Sher.