The Apollo Link is a library that provides a framework to give developers complete control over GraphQL request control flow and result resolution, allowing them to create custom clients with specific behavior. The library contains a collection of Links that perform discrete functions, such as HTTP requests, subscriptions, and modifying request control flow, which can be composed together to create a client with custom behavior. Each link in the chain applies its modification to the request's control flow, resulting in a GraphQL ExecutionResult. To support diverse behavioral requirements, each response is represented by an Observable that exposes a subscribe method with callbacks for next, error, and complete events. The Apollo Link ecosystem is community-driven and constantly evolving, providing many basic links that can be combined and extended with custom links to create the desired behavior.