Casual, shortened form of ありがとう. Used in informal speech among friends, family, or close acquaintances.
See also: ありがとう
Thanks for helping me!
プレゼントありがとね。
Thanks for the present.
ありがとうございます is the polite/formal counterpart used with strangers or superiors, while ありがと is casual slang for close relations.
Colloquial shortening of ありがとう, from 有り難く (arigataku), the adverbial form of 有り難い (arigatai, 'grateful').