Courier is a multi-channel notification platform that provides a toolbox of rich notification features suitable for various use cases. It integrates seamlessly with both mobile and web platforms, allowing developers to send SMS, email, push, and in-app notifications with a single API call. Courier connects with Segment, a customer data platform, enabling the unification of customer data into a single data store. This integration allows developers to set up automated push notifications based on transactional events from Segment events. The process involves creating a Firebase project, setting up a user in the Courier dashboard, and connecting the two platforms using a JSON file and JavaScript code. Once connected, developers can trigger push notifications using Courier automations, which can be configured to send notifications based on specific conditions or events.