The main differences between shared hosting and Virtual Private Server hosting are:
Control - A VPS account gives root access to the system, allowing for modifications of server configuration files and installation of applications. In a shared environment, the server settings are pre-configured and fixed for all clients.
Isolation - A VPS account provides a complete system resource isolation, meaning the resources you receive with the VPS platform are dedicated to you and are fully available to you at all times.
Performance - While a shared environment is a great value for applications with minimal to average system requirements, a VPS platform is best suited to a growing on-line business with growing site traffic, additional application needs, and overall more intense resource requirements. In a shared environment you are paying for a fractional use of resources that are not dedicated to you and are spread across a large group of users. If the demand of other user accounts increases, resources for your account diminish, thereby diminishing performance on your site. Purchasing VPS, at a fraction of the cost of a stand alone server, you can purchase dedicated system resources and reliability required by a growing business that needs a scalable solution.