An API is a set of tools and services that allow for easier integration between two different applications. It can be used to connect an eCommerce site with other platforms like accounting software or customer relationship management systems, or it can provide data about the business to outside parties, such as marketing agencies. The API can also be used to integrate with third-party services, such as social media platforms or advertising networks. For example, Facebook has an API that allows developers to build apps that use Facebook's data and features in their own applications.