What does arigato gozaimasu?

doomo means "very" and arigatoo means "thank you"

gozaimasu makes it "formal".

If you were thanking your sensei/someone you respect or someone you do not know very well you would say doomo arigato gozaimasu.

for just your friends, doomo arigatoo or arigatoo would suffice.

When writing these phrases in Japanese make sure you use kanji for formal and hiragana for informal. doomo arigatoo gozaimasu would be written using kanji, and when addressing a friend you would use hiragana.