Simple Notification Service - AWS messaging service for sending notifications to multiple subscribers. Like a broadcast system that can simultaneously send messages to phones, emails, and other applications.
An e-commerce site uses SNS to notify customers via SMS and email when their orders ship.
All provide publish/subscribe messaging for fan-out delivery. SNS is optimized for topic-based notifications and easy fan-out to many endpoints (SQS, Lambda, HTTP/S, email, SMS, mobile push). Azure Event Grid focuses on event routing from Azure resources to handlers; GCP Pub/Sub is a general-purpose pub/sub messaging backbone; OCI Notifications provides topic-based notifications to email, SMS (where available), and functions/streams integrations.