What is a VPS?

[roald]

A virtual private server (VPS), also referred to as Virtual Dedicated Server or VDS is a method of partitioning a physical server computer into multiple servers such that each has the appearance and capabilities of running on its own dedicated machine. Each virtual server can run its own full-fledged operating system, and each server can be independently rebooted.
The practice of partitioning a single server so that it appears as multiple servers has long been common practice in mainframe computers, but has seen a resurgence lately with the development of virtualization software and technologies for other architectures.
Virtual private server hosting

A growing number of companies offer virtual private server hosting, or virtual dedicated server hosting as an extension for Web hosting services. Some web hosting companies call a Virtual Private Server a Virtual Dedicated Server/Dynamic Dedicated Server or the other way around.
Managed Hosting

Tools are provided to monitor and control the machine.
Unmanaged Hosting

Typically only minimal services are provided, such as ssh console and reboot. Unmanaged VPS hosting is generally less expensive than managed VPS hosting.
Unmetered Hosting

Similar to unmanaged hosting but a fixed bitrate is offered so that it is not possible to exceed a monthly budget.

[seanbarron]

IMO, unmanaged VPS is intended for those users who can administrate the VPS using their technical knowledge.