ESP - IPSec Mode
ESP (Encapsulating Security Payload)
ESP provides all four security aspects of IPSec. These are confidentiality, integrity, origin authentication, and anti-replay protection. Confidentiality would ensure data is encrypted. Providing integrity would ensure data in transit has not been tampered with and origin authentication would ensure the remote peers are who they claim to be. Anti-replay will ensure duplicated traffic is not accepted which would prevent DOS attacks, as well as spoofed traffic.
ESP can operate in either tunnel mode which is more secure due to encrypting the routing, header information and IP payload, or can operate in transport mode in which it only encrypts the IP payload. Tunnel mode is usually used between gateways through the internet, and transport mode is usually used for host to host VPN’s such as between a server and a computer.In a nutshell ESP is a security protocol used with IPSec which provides source authentication, confidentiality and message integrity.
Wikipedia's guide to IPSec