# Amazon Events Reference
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An *event* indicates a change in an environment or service. For example:
+ Amazon EC2 generates an event when the state of an instance changes, such as from pending to running.
+ Amazon CloudFormation generates an event when it creates, updates, or deletes a stack.
+ Amazon CloudTrail publishes events when you make API calls.

For a list of Amazon services that generate events, see [Events](events.md).

## Using events to connect Amazon services
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Amazon EventBridge is a serverless service that uses events to connect application components together, making it easier for you to build scalable event-driven applications. Event-driven architecture is a style of building loosely-coupled software systems that work together by emitting and responding to events. Using EventBridge to deliver Amazon events from one service to another enables you to create EDA applications.

Here's how it works:

Many Amazon services generate and send events to the EventBridge default event bus. (An event bus is a router that receives events and delivers them to zero or more destinations, or *targets*.) Rules you specify for the event bus evaluate events as they arrive. Each rule checks whether an event matches the rule's *event pattern*. If the event does match, the event bus sends the event to the specified target(s).

![\[Amazon services send events to the EventBridge default event bus. If the event matches a rule's event pattern, EventBridge sends the event to the targets specified for that rule.\]](http://docs.amazonaws.cn/en_us/eventbridge/latest/ref/images/eventbridge-integration-how-it-works.png)


For more information, see [Event buses](https://docs.amazonaws.cn/eventbridge/latest/userguide/eb-event-bus.html) and [Rules](https://docs.amazonaws.cn/eventbridge/latest/userguide/eb-rules.html) in the *EventBridge User Guide*.