What is a supermarket?
A supermarket is the same as a grocery store. It is a big shop with a range of food, clothes, DVDs, technical items, homeware, bakeries, sometimes opticians, fresh meat and fish, toys, furniture, mobiles...
People mainly shop at supermarkets for a particular item such as e.g. a certain size kitchen utensil/fresh meat and fish/items which can't be bought at a local mini-store.
The word supermarket is a common noun, a general word for a type of retail store. A proper noun is the name of a specific person, place, or thing. A proper noun for the common noun supermarket is the name of a supermarket; for example: Walmart Supercenter Kroger National Supermarket Association (NSA), Flushing, NY "Supermarket", science-fiction novel by Brian Wood