What is 'Beher' ?  
'Beher' is the 'meter' of the Sher's. It can be considered as the length of the Sher. Both the lines in the Sher *MUST* be of same 'Beher'. And all the Sher's in one Ghazal *MUST* be of the same 'Beher'. There are 19 (!!) kinds of 'Beher'. But in simple terms,'Beher' is categorized in 3 classes. Short, medium, long.

Small :  
ahale dairo-haram reh gaye tere deewane kam reh gaye
Also Talat song, "dil-e-nadan tuze hua kya hai" ]  
Medium :  
umr jalwo me basar ho, ye zaruri to nahin har shab-e-gam ki seher ho, ye zaruri to nahin[ And by Gulzar, "ruke ruke se kadam, ruk ke baar baar chale" ]  
Long :  
ai mere humnashin, chal kahin aur chal, is chaman me ab apanaa guzaaraa nahin baat hoti gulon ki, to seh lete hum, ab to kaaton pe bhi haq hamaaraa nahin

So Ghazal is a collection of Sher's of SAME 'Beher'.

What is 'Radif' ?
In a Ghazal, second line of all the Sher's *MUST* end with the *SAME* word/s. This repeating common words is the 'Radif' of the Ghazal.
In our example, the 'Radif' is "nahin aati".