For Example
     - SELECT CONCAT('Naveen',' Aggarwal')  
 
which results to Naveen Aggarwal
The same can be done using + in the earlier versions
     - SELECT 'Naveen'+' Aggarwal'  
 
which results to Naveen Aggarwal
But there is a advantage of CONCAT function over '+' in sql server 2012 
     - SELECT 'Naveen'+' Aggarwal'+NULL  
 
When you execute the above, the result is NULL
But the CONCAT function will ignore NULL values
     - SELECT CONCAT('Naveen',' Aggarwal',NULL)  
 
The result is Naveen Aggarwal
So by using CONCAT function,we need not to worry about handling NULL values.
I hope u like it.