The literal meaning of Ghazal is conversing with women. Not surprisingly ghazal mostly talks about love. They may even deal with other human emotions but its love that holds center stage.
In Arabia a form of poetry called qasida was used to praise its rulers. When qasida came to Iran, a part of it known as the tashib; was separated and developed over a period of time into the ghazal.
Ghazal came to India with the Muslim culture from 12th century onwards. In those days Persian was the language used in Moghul courts but down south the Muslim rulers started using Urdu. Wali Deccany, a great Urdu poet of those times from Southern India visited Delhi and introduced Urdu to the people there. They came to appreciate the language due to its rich and beautiful expression. Soon Persian was replaced by Urdu and ghazals flourished in this language in India.
Ghazals are composed of about twelve couplets seldom more than that. Ghazals are rhyming in nature. The tone for the ghazal is set in the opening couplet. It is the representative couplet. The last couplet of a ghazal often carries the poets own view point, beliefs, emotions towards a particular subject etc. The thoughts used in the different couplet of a ghazal may vary but still each couplet stands as an independent entity with its complete thought structure.
Earlier musical forms of the ghazal shared close resemblance to Hindustani music but in recent times they have acquired the shape of film music with fast rhythms and all.