I've heard Doumo Arigatou means "Thanks a lot". We use it in Japanese when we are a little more thankful than Arigatou.

Most Japanese are insensible of difference of with "どうも" and without "どうも". We use "どうも" as "Thanks" instead of "どうもありがとう", it's just an abbreviation. Actually, I don't know the meaning of "どうも", but if I translate these sentence, "どうもありがとう" is "Thank you very much". "ありがとう" is "Thanks".

I already knew what Arigatou means. But what about Doumo Arigatou?

Arigatou means thank you and arigatou gozaimasu means also thank you . But arigatougozaimasu is very polite .And we don't use it when we talk with friends.