"Thank" is an ordinary verb, but it has two special polite uses. As an ordinary verb, it takes endings of -s, -ed, and -ing like most verbs. It means to express gratitude:
Jill forgot to thank Bill for giving her a ride from the airport.
Jill is thanking Bill for the ride right now.
Jill thanked Bill one day after he gave her a ride.
Jill often thanks people late.
The polite forms are themselves expressions of gratitude. Each is a whole sentence:
Jill said "Thanks" to Bill.
Jill said "Thank you" to Bill.
As an expression of gratitude, you can end the sentence with either an exclamation point or a period. The exclamation point indicates saying it louder; it suggests more enthusiasm and less formality. "Thank you" suggests a little more gratitude than "Thanks."
Notice that "Thank" (without -s at the end) is not one of these special polite forms.