Version 4.1 - 1.6.a GRE
GRE is Generic Routing Encapsulation. The Cisco Documentation for GRE has quite a bit of information. Let's sort through some of this and make it manageable.
GRE is a carrier protocol (the train in my previous post) and can carry any OSI Layer 3 protocol over an IP network. It is stateless and does not (itself) provide any protection. Once you build a GRE tunnel, you can use IPsec for protection, but the tunnel itself does not provide any protection.
Under the "Design" section, you may want to read about How GRE Keepalives Work, but I wouldn't focus too much on the specifics. Just know that keepalives exist. I think the more important link on the GRE page would be GRE Tunnel Interface States and What Impacts Them.
GRE is IP protocol 47. And normally when people refer to GRE, they can either be talking in general or specifically about point-to-point GRE. Point-to-multipoint GRE is normally referred to as mGRE. You will see this as MPLS VPN over mGRE.
The link above illustrates the point that GRE is a basic building block of other things. I'm going to look at the blueprint and realize that it's easier to talk about GRE as a part of the later specific topics. This is getting a wink and a promise and I'm moving on to NHRP. Realistically, if I were to create a question on this topic, it would involve knowing that GRE is protocol 47 and not port 47.