An ecommerce website or app is a storefront that allows a business to sell products or services to customers over the internet. It typically displays and describes products or services, enables customers to purchase the products or services, and accepts payments and issues returns.
Ecommerce-specific interactions like purchases, add to carts, and product views aren't sent by Google Analytics by default. To measure these interactions, which are critical to understanding the performance of an ecommerce site or app, you must implement ecommerce events.