X-as-a-Service (XaaS) is the generic version of the three cloud computing models: Infrastructure-as-a-Service (IaaS), Platform-as-a-Service (PaaS) and Software-as-a-Service (SaaS).1
The fundamental business model at the heart of XaaS is the service rental model. You rent what you need or want. 
Innovation and deployment are faster with lower maintenance costs.
Infrastructure-as-a-Service (IaaS) refers to providing the fundamental building blocks of computing that can be rented: physical or virtual servers, storage and networking.1
This is the provisioning of what can be thought of as the hardware. You need it but you cannot do anything useful with it yet.
This layer is the foundation upon which the other services are built. Think of it as online shopping for computing with virtually instant delivery. Deployments are faster with lower maintenance costs.
The IaaS layer is what is being talked about when you here the terms public vs private cloud.
Platform-as-a-Service (PaaS) is the next layer up from IaaS. In addition to the infrastructure layer of servers, storage, and networking, additional tools and software are added. The additions would include operating systems, database management, middleware, (services provisioned to applications beyond the operating system), and development tools.3
This layer is primarily for developers and operational professionals. Very few business people interact with this layer. This is the layer where you put the code and it will run. This allows companies to build applications and solutions, with control over the parts, without necessarily having the technical expertise to put it all together.
Software-as-a-Service (SaaS) is the user facing delivery of application-as-a-service. This is the version of cloud computing most people experience. The underlying hardware (infrastructure) and software (operating system) are irrelevant to the end user because the service is accessed using a web browser or app.
SaaS is a packaged service that looks the same to all users. You don’t control updates or upgrades – it just happens. There are often some level of customization or personalization, but the entire stack is managed for a third-party.
Sources:
1 Ranger, Steve, What is cloud computing? Everything you need to know about the cloud, explained. 13 Dec. 2018, https://www.zdnet.com/article/what-is-cloud-computing-everything-you-need-to-know-from-public-and-private-cloud-to-software-as-a/
2 Wikipedia.org. Cloud computing. 7 Jan 2019. https://en.wikipedia.org/wiki/Cloud_computing. Accessed 8 Jan. 2019.
3 Forrest, Conner. SaaS, PaaS, and IaaS: Understand the Differences. ZDnet, ZDnet, 7 Nov. 2017, www.zdnet.com/article/saas-paas-and-iaas-understand-the-differences/
Evans, Bob. “Microsoft Cloud Hits Superscale As Huge Customers Migrate Mission-Critical SAP Workloads To Azure.” Forbes, Forbes Magazine, 1 Mar. 2018, www.forbes.com/sites/bobevans1/2018/02/28/microsoft-cloud-hits-superscale-as-huge-customers-migrate-mission-critical-sap-workloads-to-azure/#5780f5905993.
You probably are using some cloud services today – Google Apps, Office 365, line of business applications like Salesforce or other CRMs, ERP systems, accounting, or email.
Moving more to the cloud is a big decision which depends on how you work and what you are hoping to achieve. This decision should be part of your technology strategy. The decision needs to support your business goals, your desired business processes, and your team.
The cloud is not right for every business and anyone recommending you transition should be asking tons of questions to deeply understand how you work and what you want to achieve.
Our goal is to ensure you have the right information to evaluate the alternatives. On-premise, in the cloud or a combination – the right solution for you is the one that enables your team to achieve your strategic goals and deliver on your customer promise. That is what we will help you implement.
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