The most common expression of gratitude in Japanese. Used in casual and polite situations; more formal variants include ありがとうございます and ありがとうございました.
プレゼントをありがとう。
Thank you for the present.
手伝ってくれてありがとう。
Thanks for helping me.
ありがとう is the casual, everyday form used with friends or family, while 有難うございました is the formal past-tense version used in polite or business contexts.