The difference between an API and a software library lies in their usage and purpose. An API (Application Programming Interface) specifies how software components should interact, offering a set of rules and protocols for querying and manipulating data. It acts as a communication interface between different software systems. In contrast, a software library is a collection of pre-written code that developers can use to optimize functionality without having to write code from scratch. While an API enables interaction with external systems, a software library provides resources to implement specific tasks or features within the same codebase.