Private IP addresses can be used on a private network, but they are not routable through the Internet. This is designed for the purpose of creating a measure of well-needed security, but it also conveniently saves valuable IP address space.
If every host on every network had to have real routable IP addresses, we would have run out of IP addresses to hand out years ago. But by using private IP addresses, ISPs, corporations and home users only need a relatively tiny group of bonafide IP addresses to connect their inside networks and get along just fine.
To accomplish this task, the ISP and the corporation–the end user, no matter who they are –need to use NAT(Network Address Translation), which basically takes a private IP address and converts it for use on the Internet.