VPS has a dedicated IP address, which you do not have to share with other users. This is important, as in shared hosting many domains are under one IP address and if one website fails, the whole IP address is added to the blacklist. Also, in case of shared hosting, your website's load can be affected by another user on the same server. It means that if your "neighbor" website is loading or it is inaccessible for various reasons, your website loading time can be increased too.
Most VPS hosts only offer servers running Linux-based operating systems; you'll need to dig a bit to find Windows-based VPS hosting. This is important to note if you're planning on running software that requires a Microsoft-compatible environment. That said, Linux-based VPS hosting will save you a few bucks; Linux servers usually cost $10 to $20 less than Windows servers.
Technical knowledge: No technical knowledge is required for running a managed server. When you need to do something technical all you have to do is contact support and let the skilled technical support operators handle the rest. With an unmanaged server you need to have server administration skills and love working in the command line otherwise it will be very difficult to administer your server efficiently.
The force driving server virtualization is similar to that which led to the development of time-sharing and multiprogramming in the past. Although the resources are still shared, as under the time-sharing model, virtualization provides a higher level of security, dependent on the type of virtualization used, as the individual virtual servers are mostly isolated from each other and may run their own full-fledged operating system which can be independently rebooted as a virtual instance.