Cisco definition:

A VPN(Virtual Private Networks) is a generic term that describes any combination of technologies that can be used to secure a connection through an otherwise unsecured or untrusted network .

or

It allow users from different locations to interconnect with each other , securly through the internet.

Types of VPNs

  • Site to site
  • Remote access

Site-to-Site VPN

It connect entire networks to each other.All traffic is sent and received through a VPN “gateway.” Or like  point-to-point except that there is no need of expensive  “dedicated” line.

VPN gateway device (can be router,firewall, vpn concentrator or Cisco ASA)    is responsible for encapsulating and encrypting  outbound  traffic for all traffic from a particular site to the destination site and other side gateway is responsible to decrypts the traffic and forwards it to to the private network.

Each site has it’s own internet connection which may not be from the same ISP or even the same type. One may have a T1 while the other only has DSL. Unlike point-to-point, the routers at both ends do all the work. They do all the routing and encryption. This is an easy way to connect two offices without having each user “dail-up” using a PPTP connection. Site-to-site VPNs can work with hardware or firewall devices.

Remote access VPN

They enable the individual user to connect to the company private network through the secure means/way.Client can use any sort of connectivity method.