Tamari is a particular variety of rich, dark soy sauce from Japan. Tamari is sometimes called miso-damari, a reference to the fact that it is in fact a by-product of miso making. The dark liquid produced by miso paste as it ages. Unlike most soy sauces, tamari is made with little or no wheat. Because of this, tamari is often marketed as a wheat and gluten-free sauce (though certain brands or varieties may have a small amount of wheat).
It is worth noting that tamari is not simply a gluten-free soy sauce (nor is it always gluten-free); because of the ingredients and the way it is produced, it is richer, darker, a little thicker, and more mellow than many conventional soy sauces. It’s also worth noting that soy sauces in general can vary enormously in colour and flavour.
Other names: Tamari is the only term commonly used for this sauce, though you may occasionally see it referred to as tamari soy sauce or the aforementioned miso-damari. In Japanese, tamari is たま while miso-damari is 味噌溜り.