What is an event-driven architecture (EDA), contrasted with tightly-coupled service calls?

In an event-driven architecture, components communicate by publishing and subscribing to events, rather than by making direct, synchronous calls to one another.

In a tightly-coupled design, service A calls service B directly and waits for a reply — A must know B exists, know its address, and block until B responds. In an EDA, service A instead publishes an event ("interest is due", "state changed") to a broker, and any service that cares subscribes to it and reacts on its own schedule. A never needs to know who is listening. This publish/subscribe indirection is the defining move of EDA: communication happens through events, asynchronously, instead of through hard-wired request/response chains.

Go deeper:

• Event-driven architecture (Wikipedia) — event producers and consumers, the mediator/broker topology, and why loose coupling scales.