Three main layers of Cloud computing are as follows:

Infrastructure as a Service (IAAS): IAAS providers give low-level

abstractions of physical devices. Amazon Web Services (AWS) is an example

of IAAS. AWS provides EC2 for computing, S3 buckets for storage etc.

Mainly the resources in this layer are hardware like memory, processor speed,

network bandwidth etc.

Platform as a Service (PAAS): PAAS providers offer managed services like

Rails, Django etc. One good example of PAAS is Google App Engineer. These

are the environments in which developers can develop sophisticated software

with ease.

Developers just focus on developing software, whereas scaling and

performance is handled by PAAS provider.

Software as a Service (SAAS): SAAS provider offer an actual working

software application to clients. Salesforce and Github are two good examples

of SAAS. They hide the underlying details of the software and just provide an

interface to work on the system. Behind the scenes the version of Software can

be easily changed

https://docs.oracle.com/en-us/iaas/Content/cloud-adoption-framework/cloud-computing-layers.htm#:~:text=To%20define%20a%20cloud%20strategy,as%20a%20service%20(SaaS).