Definition of thanks
1 : kindly or grateful thoughts : gratitude.
2 : an expression of gratitude return thanks before the meal —often used in an utterance containing no verb and serving as a courteous and somewhat informal expression of gratitude many thanks.
gratitude.
recognition.
acknowledgment.
benediction.
blessing.
credit.
grace.
gratefulness.
Thank you shows our appreciation and conveys our gratitude. But more than that, it is a sign of respect to the person who has helped you (or given you something). It is an indication you do not take them for granted, and an acknowledgement that they matter. And that is why saying thank you matters.
Thanks (noun) is an expression of appreciation of an action of another person. It is almost always found in the plural, rarely in the singular. "Thank you!" is always correct. "Thanks!" is slightly informal, something not said in formal situations.