Ghazal aur Nazm
Ghazals and Nazms, both are forms of poetry..
The format or the structure is different in two forms.
Ghazals are usually sung while Nazms are usually read.
Ghazal has a strict format as it has to have a Meter, Qaafiya (rythming pattern) and Radeef (the word is repeated at the end of line), while nazms are free form poetry with no such restriction. Nazms are focussed to a theme or topic while ghazal is a collection of coupletes, each of the couplet can have a different theme or independent provided they have same Meter, Qaafiya and Radeef.
For example:
Hum hain mataa-e-koocha o bazaar ki tarah
Uthati hai har nigaah kharidaar ki tarah
"ki tarah" is Radeef while "bazaar and kharidaar" is Qaafiya.

Apart from Qaafiya and Radeef, there are Matla and Maqta. Matla is the opening couplet of ghazal and usually its often repeated between other couplets. The above couplet is matla. Maqta is the usually the last couplet of ghazal, where the poet puts his signature in the sher (couplet) you can find maqta'a in many ghazal where poets name or takhallus appear in the couplet. Like in the song above the last stanza is as below:
Majrooh likh rahe hai woh ehday wafa ka naam.
this couplet is maqta as Poet has used his name in the couplet.