IaaS:
Infrastructure as a Service
Only provides virtual machine,network and security, user has to install OS and software by themselves.
The basic one.

PaaS:
Platform as a Service
Also provides database additionlly, but user has to manage and mantainence the database by themselves.

DaaS:
Database as a Service
Database is managed for user.

SaaS:
Software as a Service
User only needs to subscribe, no need to know how the infrastructure is managed.