A supermarket is a very large grocery store. Nowadays, most grocery stores in the USA are supermarkets, so the words are often used as synonyms. PS: I think the original supermarkets were called "super" because they had a bakery section, and a meat section and a fish section, and a produce (fruits and vegetables), all in one store. Traditionally, those were separate stores, and still are in some small towns and certain neighborhoods.

I agree with everyone else's comments below. Just to add my penny's worth - "store" is unlikely to be used in the UK, but very much so in the US. In the UK we use "shop". "Shop" as a noun feels very much like a a local and small-ish place to buy food items just like "store". Supermarket is, as mentioned by others, a much bigger unit with a much larger range of food and non-food items.

Most people are using them interchangeably. However, grocery stores sell food and drink items and very few other household products. On the other hand, Supermarkets offer a wider range of products for sale.

In grocery store you can buy something like rice, oil etc. But not everything. But in supermarket people can buy everything including clothes food vegetables etc. that is the difference between them.

Grocery stores stock their shelves with food and drink items, and maybe a handful of household items at most. Supermarkets, however, have a wider assortment of things for sale, including clothing, baby items, and medications etc.