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+
+# CloudEvents Concepts
+
+This document provides context for the [CloudEvents spec](../spec.md). In the
+text below there are links to definitions in the specification, followed by
+narrative text which is less precise than the formal specification, yet may
+be more approachable to newcomers.
+
+An [event](../spec.md#event) includes context and data about an [occurrence](../spec.md#occurrence).  Each *occurrence* is uniquely identified by the data
+of the *event*.
+
+*Events* represent facts and therefore do not include a destination, whereas
+messages convey intent, transporting data from a source to a given destination.
+
+## Eventing
+
+Events are commonly used in server-side code to connect disparate systems where
+the change of state in one system causes code to execute in another. For
+example, a source may generate an event when it receives an external signal
+(e.g. HTTP or RPC) or observes a changing value (e.g. an IoT sensor or period of
+inactivity).
+
+To illustrate how a system uses CloudEvents, the simplified diagram below shows
+how an event from a [source](../spec.md#source) triggers an action.
+
+![alt text](img/source-event-action.png "A box representing the source with
+arrow pointing to a box representing the action. The arrow is annotated with 'e'
+for event and 'protocol'.")
+
+The source generates a message where the event is encapsulated in a protocol.
+The event arrives to a destination, triggering an action which is provided with
+the event data.
+
+A *source* is a specific instance of a source-type which
+allows for staging and test instances. Open source software of a specific
+*source-type* may be deployed by multiple companies or providers.
+
+Events can be delivered through various industry standard protocols (e.g. HTTP,
+AMQP, MQTT, SMTP), open-source protocols (e.g. Kafka, NATS), or platform/vendor
+specific protocols (AWS Kinesis, Azure Event Grid).
+
+An action processes an event defining a behavior or effect which was
+triggered by a specific *occurrence* from a specific *source*.  While outside
+of the scope of the specification, the purpose of generating an *event* is
+typcially to allow other systems to easily react to changes in a source that
+they do not control. The *source* and action are typically built by different
+developers.  Often the *source* is a managed service and the *action* is custom
+code in a serverless Function (such as AWS Lambda or Google Cloud Functions).
+
+
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