When must I use 'thanks' and when 'thank you', which is the difference?
Here is an example:

when somebody tell me 'How are you' I sould say 'thanks' or thank you'

Question

When must I use 'thanks' and when 'thank you', which is the difference?
Here is an example:

when somebody tell me 'How are you' I sould say 'thanks' or thank you'

Answer

"Thanks"- for casual usage
"Thank you" - for formal usage

Answer

"Thank you" is much more professional then the casual "Thanks", I think also "Thank You" gets the point across how thankful you truly are.
It depends on your relationship with someone. 
If you have a very professional relationship, then you should say "thank you". But if you have casual relationship, "thanks" is just fine.

Answer

They mean the same thing and are interchangeable.  However, "Thank you" sounds slightly more formal.

Answer

"Thanks" is the simpler version of "thank you".

Answer

Hi... Don't worry the meaning is the same...