Subscribing to Platform Events | Platform Events Developer Guide

Subscribe to Platform Event Notifications with Apex Triggers
Use Apex triggers to subscribe to events. You can receive event notifications in triggers regardless of how they were published—through Apex or APIs. Triggers provide an autosubscription mechanism. No need to explicitly create and listen to a channel in Apex.

Subscribe to Platform Event Notifications with CometD
Use CometD to subscribe to platform events in an external client. A Java library and app, EMP Connector, implements all the details of connecting to CometD and listening on a channel. Use EMP Connector to subscribe to platform events easily. EMP Connector hides the complexity of subscribing to events.

Obtain Apex Trigger Subscribers
View a list of all triggers that are subscribed to a platform event by using the Salesforce user interface or the API.