With unmanaged cloud VPS hosting, the hosting provider is only responsible for the physical server and the availability of the server. This means that with the exception of the initial OS installation, the user is responsible for the maintenance, upgrades, installation of software, etc. If you choose unmanaged cloud VPS hosting, you should be familiar with the operating system and server software you choose to use.
A VPS runs its own copy of an operating system (OS), and customers may have superuser-level access to that operating system instance, so they can install almost any software that runs on that OS. For many purposes they are functionally equivalent to a dedicated physical server, and being software-defined, are able to be much more easily created and configured. They are priced much lower than an equivalent physical server. However, as they share the underlying physical hardware with other VPSes, performance may be lower, depending on the workload of any other executing virtual machines.