VPS hosting has a limit to scaling depending on the physical host machine. You have access to some extra memory and CPU power above and beyond the amount dedicated to your virtual machine, but the physical host has a fixed amount that it can deliver. If you need more, you will have to get a new plan and may experience downtime while you migrate to a new solution.
Rather than being hosted on one single instances of a physical server, hosting is delivered on a virtual partition which draws its resource, such as disk space, from an extensive network of underlying physical servers. If one server goes offline it will have no effect on availability, as the virtual servers will continue to pull resource from the remaining network of servers.