"Thanks": Use "Thanks" in informal and casual situations when expressing gratitude. It is a shorter, friendlier version of "Thank you."
For example:
When a friend treats you to coffee: "Thanks for the coffee!"
When someone holds the door for you: "Thanks!"

"Thank you": "Thank you" is the standard, polite way to express gratitude in most situations, both formal and informal. Use it when you want to show respect and appreciation.
For example:
In a professional email: "Thank you for considering my application."
After receiving a birthday present from a family member: "Thank you for the wonderful gift."

"Regards": "Regards" is a closing sentiment often used in emails or formal letters. It is a way to show respect and best wishes.
For example:
In a business email: "Thank you for your prompt response. Best regards, [Your Name]."
In a formal letter: "With warm regards, [Your Name]."

In summary, use "Thanks" for casual situations, "Thank you" for most scenarios, and "Regards" as a professional sign-off in emails or formal letters.