Reference:

https://www.cisco.com/c/en/us/products/security/what-is-a-ddos-attack.html#~types-of-ddos-attacks

Cloud computing can be broken into the following three basic models:
+ Infrastructure as a Service (IaaS): IaaS describes a cloud solution where you are renting infrastructure. You purchase virtual power to execute your software as needed. This is much like running a virtual server on your own equipment, except you are now running a virtual server on a virtual disk. This model is similar to a utility company model because you pay for what you use.
+ Platform as a Service (PaaS): PaaS provides everything except applications. Services provided by this model include all phases of the system development life cycle (SDLC) and can use application programming interfaces (APIs), website portals, or gateway software. These solutions tend to be proprietary, which can cause problems if the customer moves away from the provider's platform.
+ Software as a Service (SaaS): SaaS is designed to provide a complete packaged solution. The software is rented out to the user. The service is usually provided through some type of front end or web portal. While the end user is free to use the service from anywhere, the company pays a peruse fee.