Hoisin sauce is essentially made from beans, usually soy. There is no standard or even a traditional recipe as it springs more from Chinese peasant culture rather than the from the tradition of the Imperial chefs, so regional variations abound. The sauce sold in bottles in the west owes more to Hong Kong manufacturers spotting a marketing opportunity as Chinese restaurants and cuisine began to take off in the west and is very much made to suit western tastes.
The Cantonese words 海鮮 hoi sin means seafood, and although it is not generally used with seafood these days, it may have originally been used for enhancing the flavour of boiled or steamed seafood, or dry noodles garnished with shrimp. Be that as it may, its most common use nowadays is as a glaze for meat, or as a dipping sauce. In the west, it may be used in stir frying, but that is not the case in authentic Cantonese cuisine. In the west, it is popularly used as a dipping sauce for spring roles.
While there are regional variations, a basic hoisin sauce will comprise soy beans, ketchup, maple syrup, red chillies, and garlic. Chinese five spice, vinegar, and sugar are also commonly added. Peking-style hoisin includes starches such as sweet potato, what, or rice, and sesame seeds.
Hoisin sauce of whatever variety is mostly associated with Cantonese communities, including those of the diaspora in Vietnam, Malaysia, and Singapore. These communities still preserve varieties that are most likely the more traditional recipes of the villages of Guangdong in Southern China and bear little resemblance to the variety concocted and bottled for the west by the Hong Kong producers.