The word virtual means not existing in the real world. In computing terms, when we talk about a virtual environment, it means the hardware of computing resources cannot be seen but their presence can be felt by abstracting through Hardware Abstraction Layer(HAL) which is in our kernel. Some softwares like Virtual Box, Vmware can abstract the hardware and provides us a virtual environment.

VPN stand for Virtual Private Network. By definition, network is just a collection of interconnected systems. In a virtual private network, network is created in remote environment which is accessible by anyone if he has the valid credentials to access that network. The main advantage of using a VPN is that it can hide identity of a person (anonymous) because the system using VPN  has the IP address of that network instead of his own network.

It is a loophole in an application which allows a normal user of that application to get those privileges which is not granted to that user.