What are private IP addresses ?

There are some IP address ranges reserved by IANA for private networks. Those IPs are commonly used for home, office and enterprise LANs.

Private IPs are not allocated to any specific organization, and they are not routable (cannot be transmitted) on the public Internet. Anyone can use these IPs in any private LAN as necessary. If such a private network needs to connect to the Internet, however, it must use a NAT gateway/router or a proxy server to translate those private IPs into a public one.

The private IPv4 ranges are as follows:
10.0.0.0 - 10.255.255.255 (16+ Million addresses, 10/8 prefix)
172.16.0.0 - 172.31.255.255 (1+ Million addresses, 172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (65536 addresses, 192.168/16 prefix)

Notes:
Public IPs are routable on the Internet, private IPs are not.
The fc00::/7 IPv6 address block has been reserved by IANA for Unique Local Addresses (ULA).
For additional information, please see RFC 1918 and RFC 4193.