Arigatou on its own is a simple, somewhat casual “thank you.” That said, most people prefer doumo arigatou or arigatou gozaimasu as their standard way of saying thanks, because both of those phrases are more polite than arigatou on its own.

When expressing thanks in Japan, choosing between Domo and Arigato depends entirely on the social context and the level of politeness required. Arigato is the standard, informal way to say "thank you" to friends, family, or those of a similar or lower social standing. On its own, it is a warm but casual expression. Domo, while often used by tourists as a shorthand for thanks, is actually an adverb that means "very much" or "indeed." Used alone, it is extremely informal—similar to saying "thanks" or even a casual "hey"—and is generally reserved for very brief interactions, like when a shopkeeper gives you change or a stranger holds a door. However, using Domo by itself to a superior, an elder, or in a professional setting can be perceived as rude or dismissive because it is an incomplete thought. For a safe and polite "middle ground," you should use Arigato gozaimasu (formal) or the complete Domo arigato gozaimasu (very formal). If you are unsure of the situation, stick to Arigato gozaimasu to show respect and ensure you aren't accidentally being overly casual with someone you don't know well.