IPsec is short for IP security, which is a manner of encrypting and authenticating Internet traffic all the way down to the IP packet level. That means that protocols such as SMTP, which are inherently insecure, can be reliably secured. Inherent to IPv6, IPSec is somewhat bolted onto the side of IPv4. But it works, and it works very well.. . .
IPsec is short for IP security, which is a manner of encrypting and authenticating Internet traffic all the way down to the IP packet level. That means that protocols such as SMTP, which are inherently insecure, can be reliably secured. Inherent to IPv6, IPSec is somewhat bolted onto the side of IPv4. But it works, and it works very well.

There are two key components to IPsec: Authentication Header (AH) and Encapsulating Security Protocol (ESP). AH provides authentication, proving the packet sender really is the sender, and the data really is what they sent. ESP encrypts the payload of the packets, and can also provide authentication services (for more info on authentication via ESP, visit the IPsec working group.)