Cloud computing is the concept of making computing resources--data or software services for example--accessible through a remote, computer network. The term has proven so difficult to define that the National Institute of Standards and Technology has released 15 versions of the definition, with the latest published in October 2009.

According to NIST:

"Cloud computing is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction. This cloud model promotes availability and is composed of five essential characteristics, three service models, and four deployment models."

In an enterprise setting, cloud computing often falls into one of three models: Public cloud, private cloud and hybrid cloud.

The public cloud is perhaps the most commercial and mainstream model. A common scenario is a public cloud that allows users to access a self-service web application, hosted by a third party, such as Google's (NASDAQ: GOOG) Gmail. A private cloud model emulates the public cloud in an internal way, allowing an enterprise to host applications or virtual machines using its own resources within the firewall. A hybrid cloud, as the name suggests, combines both internal and external providers in a cloud environment.