'Thank you' and 'thanks' are the most common ways of expressing gratitude. 'Thanks' is informal and 'thank you' is more formal.

When we say 'thank you so much' and 'thank you very much', we are emphasising how grateful we are.

Thank you very much for treating the team to lunch.

We can also use 'many thanks' when we're writing a formal letter or email.

There are other ways of saying 'thank you' and these can be used in informal situations:

•​ ​Thanks so much

•​ ​Thanks a bunch

Thanks so much for helping me with the laundry.

Thanks a bunch for lending me your study notes!

•​ ​Cheers

A: Here's the dress you wanted to borrow.

B: Great, cheers.

•​ ​Ta

A: Can you pass me the biscuits?

B: Sure you can.

A: Ta.

We can also use 'thank you' or 'thanks' when we want to politely refuse something.

A: Do you want a cup of tea?

B: No, thanks.

A: Would you like me to serve dessert now?

B: No, thank you. I am full.

(Source : australiaplus.com) 

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