Used for sending SMS or E-mail as well as push notifications: Simple Notification Service

Integrates multiple services into one.

Publishers push messages to topics, and subscribers receive notifications and take subsequent steps by subscribing to topics.

A topic can have multiple subscribers, allowing the same message from a publisher to be reused by different subscribers.

SNS+SQS Combo

Publishers push messages to topics.
SQS, as a subscriber, puts events to be processed into a queue by subscribing to topics.
The subsequent instance pulls messages from SQS for processing.
This allows different people to implement different functions, even developing in different programming languages.