union and coupling

1, "What is the basic difference between coupling and unions?"
Both of these fittings are normally only found or used in 2" (DN50) and smaller screwed or socket-weld piping. The basic difference between a coupling and a union is that a coupling is for joining pipe to pipe or pipe to a swedge. A union is made for joining and disassemble of a part of a piping system (a steam trap or control valve, etc) for maintenance or replacement.
2, "What are the advantages of ones over others?"
The union is a higher cost item than the coupling. When you are just joining pipe you would want to use the lowest cost item. When you need the ability to disassemble the piping for removal of an item then you must use the higher cost union.
3, "Where we use coupling and where we use unions ?"
Use couplings to join pipe in pipe racks. Use unions at steam trap or control valve manifolds for removal of items for repair.
4, "Is there any guidelines or code that specifies where should we use what?"
The use of couplings or unions is not a code issue.