The main features of a Cloud platform according to the NIST Standard are on-demand self-service, broad network access, resource pooling, rapid elasticiy, and measured service. The exact NIST definitions are pasted below. 

In practise, 'broad network access' means that the X a user gets at an XaaS system can be accessed over network by hundres, thousands or even hundres of thousand users at the same time. 'Measured service' in turn means that the user can check the resources he is using. 'Resource pooling' means that the provider has the capacity to server multiple (many, many) users and is able to sustain the increasing demands of customers. 

The key differentiating factors that differentiate cloud platforms from other forms of hosting providers is the on-demand self-service in combination with the rapid elasticity property. In that sense a user will not only be able to automatically request a resource without human interaction, but also to get the response that request in the order of seconds.

NIST Definitions  #

On-demand self-service: A consumer can unilaterally provision computing capabilities, such as server time and network storage, as needed automatically without requiring human interaction with each service provider. 

Broad network access: Capabilities are available over the network and accessed through standard mechanisms that promote use by heterogeneous thin or thick client platforms (e.g., mobile phones, tablets, laptops, and workstations). 

Resource pooling: The provider’s 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. There is a sense of location independence in that the customer generally has no control or knowledge over the exact location of the provided resources but may be able to specify location at a higher level of abstraction (e.g., country, state, or datacenter). Examples of resources include storage, processing, memory, and network bandwidth.

Rapid elasticity: Capabilities can be elastically provisioned and released, in some cases automatically, to scale rapidly outward and inward commensurate with demand. To the consumer, the capabilities available for provisioning often appear to be unlimited and can be appropriated in any quantity at any time.

Measured service: Cloud systems automatically control and optimize resource use by leveraging a metering capabilityat some level of abstraction appropriate to the type of service (e.g., storage, processing, bandwidth, and active user accounts). Resource usage can be monitored, controlled, and reported, providing transparency for both the provider and consumer of the utilized service