Cloud computing primary focuses on delivering computing resources over a network. The National Institute of Standards and Technology (NIST) defined it as a model for enabling ubiquitous, convenient, on-demand network access to a shared pool of configurable computing resources. More specifically, five characteristics, three service models, and four deployment models are defined.
Although there is no unique definition of cloud computing, people generally accept the term of Essential Characteristics of cloud computing include "On-demand self-service", "Broad network access", "Resrouces Pooling", "Rapid elasticity", and "Measured service".