Workload Groups in SQL Server 2012 I have been looking for an answer about "Workload Groups" in SQL Server?

What exactly it is Workload Groups? 

Could you explicit a real world situation when this term is used?

Thanks in advance your your help!
Workload groups are a concept of the resource governor feature in SQL server.  You would define workload groups (such as a prod group and a test group) on a server and then define the amount of resources that can be used by each.

So for instance if you only had one SQL server for test and prod say, you could define the 2 workload groups and limit the CPU/Memory that can be used by each, to limit the impact of the test workload on the prod workload (SQL 2014 can also limit the IOPs used by each group too).

You should read up more on the resource governor here:

Resource Governor