VPN - Virtual Private Network.
VPN is a communications network tunneled through another network, and dedicated for a specific network. One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption. VPNs, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features.
In section "VPN Overview" you can know what VPN is and about its features.
"VPN Hardware" will tell you about hardware for creating Virtual Private Networks.
In section "VPN & IPSec" you can know about the protocol which seems destined to become the de facto standard for VPNs.
Recommended books for VPN: "VPN Books"
"VPN & Security" compares the potential security vulnerabilities of a regular network connection with the potential security vulnerabilities of a VPN connection.
"VPN Software" tells about the popular VPN software applications.
And "VPN FAQ" - Frequently Asked Questions about Virtual Private Network.