Function-as-a-Service (FaaS) is a cloud computing model that allows developers to execute individual functions or code snippets without having to worry about the underlying infrastructure. Essentially, FaaS enables developers to upload and run code in the form of functions without dealing with the deployment, scaling, or management of server infrastructure.

The idea behind FaaS is that developers only need to write and upload the code that fulfills a specific function. The FaaS platform then handles the execution of this code when triggered by events or requests. A typical example of FaaS is using serverless computing in the cloud, where developers deploy functions in the cloud that run only when needed.

Popular FaaS platforms include AWS Lambda by Amazon Web Services, Azure Functions by Microsoft Azure, and Google Cloud Functions by Google. They allow developers to upload and execute code in various programming languages, simplifying application development and scalability without worrying about the underlying infrastructure.

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.

Horizontal scalability refers to a system's capability to handle increased workloads by adding more resources or hardware components, enhancing its performance. In contrast to vertical scalability, where performance improvement occurs by adding resources to a single node or machine, horizontal scalability scales by adding additional instances of resources that work together.

Typically, horizontal scalability means the system can distribute loads across multiple machines or servers. Cloud computing platforms are often designed to offer horizontal scalability, allowing resources to be dynamically added or removed as needed to enhance performance and availability.

An example of horizontal scalability is expanding a web server by adding more servers to better handle user requests, rather than just increasing the resources of the existing server.

Vertical scalability refers to a system's ability to handle increasing or decreasing workloads by adjusting its resources. In the context of computer technologies, vertical scalability generally means enhancing the performance of a system by adding or removing resources within the same hardware.

In contrast to horizontal scalability, where capacity is increased by adding more machines or nodes, vertical scalability involves improving the capability of a single device, such as a server or a database, by adding more resources like CPU, RAM, or disk space.

Vertical scalability provides a relatively straightforward way to enhance a system's performance. However, there's a limit to how much a single device can scale, constrained by its physical limitations. In some cases, scaling might hit the boundaries of the hardware, leading to bottlenecks. This is why many companies also opt for horizontal scalability to make their systems more robust and resilient.