Pub/Sub is an asynchronous global messaging service. There are three terms in Pub/Sub that appear often: topics, publishing, and subscribing.

A topic is a shared string that allows applications to connect with one another through a common thread.

Publishers push (or publish) a message to a Pub/Sub topic. Subscribers will then make a subscription to that thread, where they will either pull messages from the topic or configure webhooks for push subscriptions. Every subscriber must acknowledge each message within a configurable window of time.

In sum, a publisher creates and sends messages to a topic and a subscriber creates a subscription to a topic to receive messages from it.