The golden rule for unmanaged VPS hosting is this: If you have a problem with your unmanaged VPS Account, it is your problem. If the problem is software related, resource related, performance related, need troubleshooting or configuration of software, or need general help understanding Unix or server applications, you are on your own and should not expect help from the web host.
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.