Qafia — the rhyme
The qafia is the rhyming word — the word that changes from sheʿr to sheʿr, but always rhymes. In a ghazal, the qafia appears at the end of the second misra of every sheʿr. (In the opening sheʿr, the matla, it appears at the end of both misras.)
In Ghalib's famous ghazal, the rhyme is on nikle — and across the ghazal the rhyming words are dam nikle, kam nikle, ham nikle, sanam nikle, and so on. Each is a different word, but each shares the rhyme.
Radif — the refrain
The radif is the refrain — a word or short phrase that is repeated exactlyat the end of every sheʿr's second misra, afterthe qafia. Unlike the qafia, the radif doesn't change. It is the same word, ghazal after ghazal, sheʿr after sheʿr.
Not every ghazal has a radif. Many of the greatest do. Faiz's famous ghazal Gulon mein rang bhare has the radif chale. At the end of every couplet, after the rhyming qafia word, comes the same word, chale — like a bell ringing out at the close of every verse.
گلوں میں رنگ بھرے، باد نوبہار چلےچلے بھی آؤ کہ گلستاں کا کاروبار چلےRead both lines and feel where chale lands. That recurring refrain is the radif. The slot before it, where the rhyming word changes from couplet to couplet, is the qafia.