Cloud computing provides on-demand, scalable access to shared computing resources via the internet. It has three main service models - Software as a Service (SaaS), Platform as a Service (PaaS), and Infrastructure as a Service (IaaS). Major cloud providers include Amazon (IaaS), Microsoft Azure (PaaS), and Salesforce (SaaS). The document discusses key characteristics, deployment models, security aspects, and the promise of cloud computing for enabling flexible, efficient, and scalable IT services.