Learn in the textbookCloud Service Models (○aaS)
DefinitionCloud computing (クラウドコンピューティング) is a way of using IT resources such as servers and software that are set up across the network (usually in a provider's facilities), drawing on them only when you need them and only as much as you need.
ExplanationCharges are usually based on how much or how long you use the service, which makes it easier to keep the up-front cost of buying equipment low. Offerings are grouped by how many layers the provider supplies, and each such type is called a service model (サービスモデル). A setup where a company places its information systems in facilities it owns and manages, and runs them itself, is called on-premises (オンプレミス).
AnalogyIt is like hooking up to the water mains instead of digging your own well, then paying for however much comes out of the tap.
Memory hookNetwork diagrams have long drawn the internet side as a cloud shape, so "cloud" became the name for using resources without caring where they actually sit.