Ghazal is an ancient form of poetry which originated in Arabia, spread to ancient Persia and was adopted by the earliest classical Urdu poets of Moghul India.
The ghazal poetic format is both liberating and restrictive in its format!! Each couplet's theme or topic or idea is unrelated to any other couplet in the Ghazal. However, both lines of the first couplet must rhyme to each other and after that, the second line of each couplet must rhyme with the first couplet.
It was customary to have the Ghazal writing poet's assumed literary name (Takhallus or non de plume or pen name) . . . which was a single word always such as Meer or Ghalib or Momin . . . to be incorporated into the last couplet of the ghazal like the artist's signature at the bottom of a painting. This is kind of like the famed movie director Alfred Hitchcock putting in a token appearance in at least one scene of every movie he ever directed !! Some modern Urdu poets have done away with this classical tradition of having a pen name and incorporating it into the last couplet of the ghazal, notably Majeed Amjad (who omits using it in many of his Ghazals) and Parveen Shakir.
So, in a sense, the Urdu Ghazal competes in the level of restriction on verbal creativity with the Japanese Haiku ... since the entire idea must be conveyed in two lines which either must rhyme to each other or where the second line must rhyme with the first couplet of the Ghazal.
The Ghazal continues to flourish in the 21st Century in modern Urdu poetry ! It is fascinating for me to explore the new Ghazal diction, newer topics, new images captured by the modern masters of Urdu ghazal e.g.
منیؔر نیازی تھکے لوگوں کو مجبوری میں چلتے دیکھ لیتا ہو ُں میَں بس کی کھڑکیوں سے یہ تماشے دیکھ لیتا ہو ُں
کبھی دل میں اداسی ہو تو اُن میں جا نکلتا ہوُں پرانے دوستوں کو چُپ سے بیٹھے دیکھ لیتا ہوُں
احمد مشتاق آنکھ میں اَن کہے اَن سُنے واہمے ، کان میں گرم آغوش کے زمزمے
تھک کے سونے لگیِں سیج پر دُلہنیں، خواب ٹھنڈے دنوں کے ستانے لگے
پھر کوئی شہر آنکھوں میں پھرنے لگا، پھر مجھے راستے یاد آنے لگے
Some critics see the flourishing of the Ghazal format and the comparative paucity of newer Urdu poets who write non-Ghazal Urdu poems (either Urdu rhymed poems or Urdu free verse poems) as a sign that most new Pakistani poets cannot sustain a single, focused theme or idea for more than two lines in their poetic creations anymore!! To answer these critics, a modern poet must experiment with all poetic formats, song, ghazal, rhymed poem and free verse (as Majeed Amjad, Munir Niazi, Fahmida Riaz and Sarmad Sehbai have done successfully).