Virtualisation is a technology that allows for the creation of virtual replicas of computer resources such as servers, operating systems, and applications, enabling the production of virtual instances. These virtual instances can run on a single physical server or host, maximising hardware utilisation. By separating resources from the underlying hardware, virtualisation allows multiple virtual machines to run independently on the same physical server without interfering with each other.

Scalability refers to a system's ability to handle increasing workloads by adding resources, either manually or automatically, to accommodate the additional demand. In Azure, this means scaling out to increase capacity during peak periods and returning to normal levels when the peak subsides.

Agility refers to a cloud customer's capability to quickly develop, test, and deploy applications using cloud resources. Agility enables rapid responses to critical business needs and objectives.

High availability refers to a collection of technologies that minimise IT downtime by ensuring the continuous operation of IT services through the use of redundant, fault-tolerant components within the same data enter. High availability means that services remain uninterrupted, providing consistent up-time. In everyday terms, it’s like saying, "The power won't go out," ensuring that nothing disrupts your work.

A fault-tolerant cloud infrastructure maintains uninterrupted services by seamlessly adapting to and recovering from unexpected disruptions. This essential capability underpins the development of reliable and resilient cloud solutions, ensuring continuity and minimising downtime.

Global reach allows businesses to operate globally by storing data in the region closest to their customers which ensures efficient access and a seamless user experience.

The difference between scalability and elasticity is primarily in how they address resource adjustments. Scalability handles predictable growth, while elasticity deals with real-time, automatic adjustments.