Business-to-consumer (B2C) APIs are the APIs that power web and mobile applications. They are typically consumed by modern front-end clients to allow end-users access to the company's business functionality exposed by these APIs.
Business-to-business (B2B) APIs are those used by the company's business partners (other companies), sometimes to provide them with services (these businesses being the end customer), and sometimes to provide value to joint customers (B2B2C).
B2B APIs are fundamental to the enterprise's digital transformation, as they enable it to streamline how it works with its suppliers, resellers, and other partners, as well as provide better experience to its customers.