For specific information about WiscVPN, see it.wisc.edu/services/wiscvpn/.
Generally speaking, a virtual private network (VPN) is a private data network that makes use of a public communications infrastructure, maintaining privacy through the use of a tunneling protocol and security procedures.
The goal of VPN is to provide the same security as a private data network at a much lower cost by using the shared public infrastructure.
Using a virtual private network involves encrypting data before sending it through the public network and decrypting it at the receiving end. An additional level of security involves encrypting not only the data but also the originating and receiving network addresses. Microsoft, 3Com, and several other companies have developed the Point-to-Point Tunneling Protocol (PPTP) and Microsoft has extended Windows to support it.
Information adapted from whatis.com.