The IP Security (IPSec) Protocol is a standards-based method of providing privacy, integrity, and authenticity to information transferred across IP networks. IPSec provides IP network-layer encryption. The two prime functions of IPSec are to ensure data security and data integrity. Security is achieved through data encryption techniques, and integrity through a combination of techniques that authenticate the data sender.

Furthermore, IPSec can be used to form ‘tunnels’ through IP networks. In other words, it can make a connection between two computers or networks on the Internet appear as though they’re connected via a private link. This is known as a VPN, a virtual private network.

So, in answer to the question, "what is IPSec", it’s a mechanism for providing totally secure virtual private networks across low-cost public networks such as the Internet.