Here is a quote from another site:
True soya sauce, called shoyu, is made by fermenting soya beans with cracked roasted wheat, salt and water. Tamari is similar but slightly stronger and made without wheat (and so is gluten-free). Fermentation for shoyu and tamari takes about one year. Much of the soya sauce available in supermarkets is not true soya sauce but is made by chemical hydrolysis from defatted soya flour, caramel colouring, and corn syrup without any fermentation process.

Tamari -- A soy sauce made from soybeans, water and sea salt. It has a slightly stronger flavor than shoyu. Organic soybeans are commonly used and no MSG or artificial additives are added. It can be used in small amounts in stir fries, soups, dressings, sauces, and other dishes. Tamari can be found in most health food stores.
www.soyinfo.com

And here is a site with more than you really wanted to know about tamari:
http://www.organic.nl/reindex.html?prodinfo/19190.htm

I hear you saying "all I wanted was a little something on my cauliflower!"

Hope this helps

Colin