A polite, slightly emphatic expression of gratitude. The どうも adds a sense of 'very much' or 'indeed'.
Thank you very much for the present.
Thank you very much for the other day.
ありがとうございます is the standard polite form used in formal or business settings, while どうも有難う is a warmer, everyday emphatic thanks.
Standard kanji-kana mixed spelling; it is the primary form.
Alternate kanji spelling with 有り難う; less common but still encountered.
Combination of どうも (very much, indeed) and ありがとう (thank you). ありがとう derives from 有り難い (grateful, rare), literally 'difficult to exist'.