What is the Hypergrid?

What is the Hypergrid? « justincc’s opensim blog: "Essentially, a Hypergrid is a confederation of OpenSim systems that have enabled the Hypergrid facility. Each user has a home grid or standalone where their user profile, avatar appearance and inventory is stored. Possible homes range from a standalone on the user’s own machine up to a large OpenSim grid hosted by a third party."