XaaS or *aaS is an acronym that stands for “everything as a service” or “anything as a service”. It regroups products, tools and technologies provided to users as a shared service over a network (usually internet) rather than provide on-premise within one single company. 
The following image gives the difference between On-site, IaaS, PaaS and SaaS models and shows what you manage vs. what the service provider manages: 
Everything as a Service (XaaS)
Everthing as a Service. But what are the “as a Service” offerings? Let’s try to list’em all…
For each service, I give a definition with its source and example of services from different companies (Amazon Web Services, Citrix, Google, Microsoft, SalesForce…). BTW, find the intruder in the list!
Artificial Intelligence as a Service (AIaaS)
Artificial Intelligence as a Service (AIaaS) is a third-party offering of AI outsourcing.
Backup as a Service (BaaS)
Online backup service, also known as cloud backup or backup as a service (BaaS), is a method of offsite data storage in which files, folders, or the entire contents of a hard drive are regularly backed up by a service vendor to a remote secure cloud-based data repository over a network connection.
Container as a Service (CaaS)
Containers-as-a-Service (CaaS) is a model of cloud services computing that lets users deploy and manage applications through container-based abstraction. The provider offers the framework, or the orchestration platform, on which the containers are deployed and managed, and it’s through this orchestration that key IT functions are automated.
Desktop as a Service (DaaS)
Desktop as a Service (DaaS) is a cloud computing offering where a service provider delivers virtual desktops to end users over the Internet, licensed with a per-user subscription.
Source: [VMWare DaaS]
Amazon Workspaces, Citrix Managed Desktops, Microsoft Windows Virtual Desktop
Database as a Service (DBaaS)
Database as a service (DBaaS) is a cloud computing service model that provides users with some form of access to a database without the need for setting up physical hardware, installing software or configuring for performance.
Disaster Recovery as a Service (DRaaS)
DRaaS, also known as disaster recovery as a service, is the replication of hosting of physical or virtual servers by a third party to provide failover in the event of a man-made or natural catastrophe.
Function as a Service (FaaS)
Function as a service (FaaS) is a category of cloud computing services that provides a platform allowing customers to develop, run, and manage application functionalities without the complexity of building and maintaining the infrastructure typically associated with developing and launching an app. Building an application following this model is one way of achieving a “serverless” architecture, and is typically used when building microservices applications.
Infrastructure as Service (IaaS)
The capability provided to the consumer is to provision processing, storage, networks, and other fundamental computing resources where the consumer is able to deploy and run arbitrary software, which can include operating systems and applications. The consumer does not manage or control the underlying cloud infrastructure but has control over operating systems, storage, and deployed applications; and possibly limited control of select networking components (e.g., host firewalls).
Amazon EC2, Google Compute Engine, Microsoft VM
IoT as a Service (IoTaaS)
IoT as a service offers to assist enterprises with the deployment of Internet of Things (IoT) applications.
Mobility as a Service (MaaS)
Mobility as a Service (MaaS), aka Transportation as a Service (TaaS), is the integration of various forms of transport services into a single mobility service accessible on demand.
Source: [MaaS Alliance]
Network as a Service (NaaS)
Source: [Techopedia NaaS]
Amazon Networking: Amazon VPC, Amazon Route 53, Amazon Elastic Load Balancing, AWS WAF, AWS Firewall Manager…, Azure Networking: Azure ExpressRoute, Azure DNS, Azure Load Balancer, Azure WAF, Azure Firewall…
Pizza as a Service (PIZZaaS)
Back in 2014 Albert Baron, Enterprise Architect at Google was trying to explain the differences between IaaS, PaaS and SaaS using the example of a (virtual) Pizza as a Service model:
Platform as a Service (PaaS)
AWS Elastic Beanstalk, Google App Engine, RedHat OpenShift, SalesForce Platform (aka Lightening or force.com)
Software as a Service (SaaS)
Google Workspace (aka G Suite), Microsoft 365, SalesForce
Security as a Service (SECaaS)
- Anti-virus and anti-malware/spyware
- Business continuity and disaster recovery (BCDR or BC/DR)
- Continuous monitoring
- Data loss prevention (DLP)
- Email security
- Identity and access management (IAM)
- Intrusion detection and protection services (IDS/IPS)
- Network security
- Penetration testing
- Security assessment
- Security Information and Event Management (SIEM)
- Vulnerability scanning
- Web security
Source: [Wikipedia SECaaS]
STorage as a Service (STaaS)
If you find more examples of “as a Service” products, let me know in the comment.
 XaaS (Anything as a Service), TechTarget
 IaaS vs PaaS vs SaaS, RedHat
Why The ‘As-A-Service’ Model Works So Well For Digital Transformation, Daniel Newman, Forbes, June 27, 2007
AWS to Azure services comparison, Microsoft, November 11, 2020
Everything as a Service (XaaS), Definitions and examples, Sam Solutions, January 9, 2020
[MaaS Alliance] MaaS White Paper, MaaS Alliance, September 4, 2017
[NIST] The NIST Definition of Cloud Computing, National Institute of Standard Technology, US. Department of Commerce, Special Publication 800-145, September 2011
[MyBlindBird AIaaS] AI as a Service (AIaaS), My Blind Bird, December 17, 2019 by Vijay Soni
[Baron] Pizza as a Service, Albart Baron, Google, July 30, 2014
[NetApp BaaS] What is Backup as a Service, NetApp
[RedHat CaaS] What is CaaS?, RedHat
[Techopedia DBaaS] Database as a Service (DBaaS), Techopedia
[Techopeida NaaS] Network as a Service (NaaS), Techopedia
[Techopedia STaaS] STorage as a Service (STaaS), Techopedia
[VMWare DaaS] Desktop as a Service, VMware Glossary
[Wikipedia SECaaS] Security as a Service, Wikipedia