Oyster Sauce is a thick, salty, oyster-flavored Asian flavored condiment (sauce) called ho yau in Cantonese and mostly sold in bottles, sometimes in cans. Thai oyster sauce tends to have more of an "oyster" flavor than the products from China.
If you're wondering what can I substitute for oyster sauce, here are some good options.
Use in small amounts to add flavor to many dishes. It goes equally well with meats and vegetables. Originally it was made from oysters, salt and water and had a grey, unappealing appearance but processing techniques have improved the look and the flavor. Once opened store oyster sauce in the refrigerator.
Oyster sauce can be found in most well-stocked grocery stores, look for it in the Asian Foods section. The best brands are found in Asian markets and Lum Kum Kee Oyster Sauce is a very good brand. You can buy this brand on Amazon: