What is 'Kaafiyaa' ?  
'Kaafiyaa' is the rhyming pattern which all the words before 'Radif' *MUST* have.  
In our example the 'Kaafiyaa' is "bar", "nazar", "par", "magar" etc. This is a necessary requirement. Something which is followed even in the exceptions to all these rules. So Ghazal is a collection of Sher's of same 'Beher', ending in same 'Radif' and having same 'Kaafiyaa'.  

What is 'Matla' ?  
The first Sher in the Ghazal *MUST* have 'Radif' in its both lines. This Sher is called 'Matla' of the Ghazal and the Ghazal is usually known after its 'Matla'. There can be more than one 'Matla' in a Ghazal. In such a case the second one is called 'Matla-e-saani' or 'Husn-e-matla'. In our example, the first Sher is the 'Matla'

What is 'Maqta' ?  
A Shayar usually has an alias ie. 'takhallus' eg. Mirza Asadullakhan used 'Ghalib' as his 'takhallus' and is known by that. Other examples are 'Daag' Dehlvi, 'Mir' Taqi Mir, Said 'Rahi', Ahmed 'Faraz' etc. There is a Sher in a Ghazal, the last one, which has the Shayar's 'takhallus' in it. To summarize, Ghazal is a collection of Sher's (independent two-line poems), in which there is atleast one 'Matla', one 'Maqta' and all the Sher's are of same 'Beher' and have the same 'Kaafiyaa' and 'Radif'.