Cloud computing is the delivery of computing services (including servers, storage, databases, networking, software, analytics, and intelligence) over the Internet ("the cloud"). Instead of owning and maintaining their own computing infrastructure or data centers, companies can access these services from a cloud provider and pay only for what they use.

Key characteristics of cloud computing:

1. On-demand self-service: Users can provision computing capabilities as needed, automatically without human interaction.

2. Broad network access: Services are available over the network and can be accessed through standard mechanisms by diverse client devices.

3. Resource pooling: Computing resources are pooled to serve multiple consumers using a multi-tenant model, with different physical and virtual resources dynamically assigned and reassigned according to consumer demand.

4. Rapid elasticity: Capabilities can be elastically provisioned and released, in some cases automatically, to scale rapidly outward and inward commensurate with demand.

5. Measured service: Resource usage can be monitored, controlled, reported, and billed transparently based on utilization.

The three main cloud service models are:

1. Software as a Service (SaaS): Software applications hosted in the cloud that users access through a web browser or API.

2. Platform as a Service (PaaS): Platforms provided for developing, testing, delivering, and managing applications.

3. Infrastructure as a Service (IaaS): Virtual computing resources like servers, storage, and networking are provided on-demand.

Major cloud providers include Amazon Web Services (AWS), Microsoft Azure, Google Cloud Platform (GCP), and others. Cloud computing enables greater flexibility, cost-efficiency, scalability, and reduced maintenance overhead.