Google Cloud Pub/Sub is a managed messaging service provided by Google, based on the Publish/Subscribe model. It enables scalable and reliable message delivery between applications and systems in real-time.

Cloud Pub/Sub serves as a central intermediary for message delivery between different components within cloud infrastructure or across various applications. It facilitates Publish/Subscribe communication, where Publishers send messages to specific topics, and Subscribers subscribe to these topics to receive messages.

Some key features of Google Cloud Pub/Sub include:

1. Scalability: It can handle messages in large volumes and is designed for high throughput rates.

2. Reliability: It ensures message delivery with low latency and offers persistence to prevent message loss.

3. Real-time processing: Facilitates real-time message transmission between applications or systems.

4. Integration: Seamlessly integrates with other Google Cloud services and can connect to external systems.

Cloud Pub/Sub is commonly used in cloud-based applications, data processing pipelines, real-time analytics, IoT (Internet of Things), and other scenarios requiring reliable and scalable message delivery.

A Subscriber, in the context of a message broker or messaging system, is an entity or component that receives messages and acts upon them. While the Publisher generates messages and sends them to the message broker, the Subscriber is a part of the system that has subscribed to receive specific types of messages forwarded by the message broker.

Subscribers are recipients of messages who subscribe to particular channels, topics, or types of messages that are relevant to them. Once the message broker receives messages that match the subscription criteria of the Subscribers, it forwards those messages to the corresponding Subscribers, who can then process or respond to them.

For instance, in a messaging system, a Subscriber could be an application waiting for specific types of notifications. Once the message broker receives messages that meet those criteria, it forwards them to the respective application, which then processes or reacts to the received information.