NIST's five keys requirements of cloud computing

 1.On-demand self-service-ability of user to sign-up and receive service without    delay.

 2.Broad network access-ability to access the service through desktops,mobiles,tabs etc.

 3.Resource pooling-computing resources are pooled to serve multiple consumers using a multi-tenant model.

 4.Rapid elasticity-capability to cope with demand peaks.

 5.Measured services-utility based billing i.e subscription based.