ESP is Encapsulating Security Payload, protocol 50. Is it defined by RFC 2406.
ESP is what data is encapsulated in to be shipped between IPsec peers.
See Negotiation? for details on exactly when ESP started to get used.
NATTraversal IETF drafts encapsulate ESP packets into UDP packets, for bypassing firewalls that either block or mangle protocol 50 packets.
