Shoyu is traditional Japanese soy sauce, slowly fermented with soybeans, wheat, water and salt. Compared with tamari, which is made almost entirely from soy, it is lighter, sweeter and more fragrant, and contains gluten.
I use it to deglaze stir-fried vegetables, in dressings for noodles and crunchy vegetables, and to marinate tofu and seitan. A little is enough to add savouriness and that depth of flavour, umami, typical of fermented foods.