A qataa, very simply, is a poem of four lines---a quatrain. It may occur in the middle of a ghazal (where the poet is unable to finish a thought in two lines, and chooses to use four). It may also be a stand-alone verse, un-embedded in any long poem. Here is an example of a stand-alone qataa from Faiz:
Raat yoon dil mein teri khoi hui yaad aayi
Jaise veerane mein chupke se bahaar aa jaaye
Jaise sehraaon mein haule se chale baad-e naseem
Jaise beemar ko be-vajah qaraar aa jaaye
Translation:
Your faded memory visited my heart last night,
As if the spring came to the ruins, real quiet.
As if the zephyr silently cooled the desert,
And the sick, miraculously, gained some respite.
Which of these is not true of a qataa?