what is the significance of vpn?

A virtual private network (VPN) is a technology that creates an encrypted connection over a less secure network. The benefit of using a VPN is that it ensures the appropriate level of security to the connected systems when the underlying network infrastructure alone cannot provide it. The justification for using a VPN instead of a private network usually boils down to cost and feasibility: It is either not feasible to have a private network (e.g., for a traveling sales rep) or it is too costly to do so. The most common types of VPNs are remote-access VPNs and site-to-site VPNs. 

Name some VPN technologies supported by Windows 2000

Microsoft Windows 2000-based virtual private networking (VPN) supports Internet-industry standards technology to provide customers with an open interoperable VPN solution. Microsoft is committed to IETF (Internet Engineering Task Force) standards-track-based technology such as Internet Protocol Security (IPSec) and Layer 2 Tunneling Protocol (L2TP) as well as Point-to-Point Tunneling Protocol (PPTP)a proven published informational RFC that is supported in multiple interoperable third-party products.

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  PPTP provides simple-to-use, lower-cost VPN security. Unlike IPSec technology, PPTP is compatible with Network Address Translators (NAT) and supports both multi-protocol and multicast environments. It also combines standard user password authentication with strong encryption without requiring the complexity and expense of public key infrastructure (PKI).
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  IPSec provides advanced security for VPN but was not designed to address critical remote access requirements such as User Authentication and Address Assignment. In addition, it does not support multi-protocol or multicast (including some routing protocols). It is applicable primarily to IP-only, unicast-only situations.
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  L2TP in combination with IPSec is the only standards-track technology that addresses these remote access VPN requirements while leveraging IPSec for encryption. L2TP currently retains the same IETF standards-track status as IPSec.
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  Third-party IPSec-only implementations that do not use L2TP with IPSec are using non-standard proprietary technologies that can lock customers into closed solutions.