Hypergrid Resources Library/What is a Grid?
In keeping with our metaphor, if teleporting is like flying in a plane to multiple places on the same planet then hypergridding is like taking a rocket ship from planet to planet to travel the universe. So a Grid is like a whole other planet. What might be confusing to the newbie, is that all the other habitable planets you visit might look a lot like your home planet Earth, so much so you might forget that you're really somewhere else entirely!
Put another way, Second Life is one giant grid owned and operated by Linden Labs in San Francisco, California. The whole thing is stored on their own servers (wherever those servers might be), and they control who has an account or access etc. In Opensim there are many grids, however, hosted by all kinds of people or organizations. Each of them has their own logins, members, terms of use, etc., and they could be located anywhere in the world, in any country. You can have multiple avatars and accounts over the metaverse of course, but hypergridding allows you to travel from your “home” planet, or grid, to just about anywhere in the metaverse. You do NOT need to create an account with each different virtual world.
Additional Reading edit
http://opensimulator.org/wiki/Grid_List