GRE (General Routing Encapsulation)
Used as a simple and effective way to create a tunnel, over the public internet
It was developed by Cisco to encapsulate a wide variety of protocols over the Network Layer
GRE does not provide any encapsulation
IPsec (Internet Protocol Security)
Used to protect data flows between clients
Uses TCP for transport
IPsec provides confidentiality, integrity, authentication and anti-replay protection
Used heavily by VPN Servers