Short answer: tamari sauce is a type of soy sauce from Japan. It contains very little or no wheat. Long answer - complete guide to soy sauce.
Tamari vs Soy Sauce vs Coconut Aminos - Downshiftology
Tamari: What's the Deal with this Soy Sauce Substitute?
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What's the difference between tamari and soy sauce?
What is Tamari? (How to Use It + Difference From Soy Sauce)
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Soy Sauce - different types, when to use what, best substitutes
Tasting Soy Sauce and Sesame Oil - The Accidental Locavore