They call it THE Grid because it is definitely the earliest surviving, and possibly once the most powerful, magical network to ever be in existence on Earth. Some mages don’t bother with playing it safe by saying ‘possibly:’ while They have certainly produced networks of similar potency and subtlety, it’s arguable that THE Grid simply had more reach. Unfortunately, there’s no way to tell at this point, as THE Grid is at least 70 million years old. Which, yes, makes it of pre-human origins.
THE Grid is absolutely not active, in the sense that more modern magical networks are; whatever central power nexus or linked nexi existed back then have long since sublimed back into the aether. What’s left are largely nodes: permanent minor bits of space-time that have been folded to allow ambient magical energy to collect and accumulate. This is typically only enough to allow nodes to remain magically entangled with each other, which effectively allows mages to use them to communicate cheaply with other mages in the presence of a node.
Nodes of THE Grid thus tend to attract magic-users, which means that they eventually attract magical organizations. Communication via THE Grid is secure, in the sense that nobody can jam a transmission — but it is also heavily insecure, since anybody at a node can listen in to anybody else’s transmission. Mages can still use codes or obfuscation magic; but what one mage can do, another can undo. Many wizardly orders simply don’t bother using a node to say anything that they wouldn’t want the rest of the magical world to hear anyway. Heck, one group of magicians in Toledo use their node to ‘broadcast’ a very popular music/magical news channel.
What makes this all different from, say, using the Internet is that THE Grid seems to have some kind of limited (very limited) repair function. They say (the oldest of Them, at least) that THE Grid was notably less powerful even a century ago. Subjective memories asid, it does certainly appear that regular use of a node increases both its capacity for collecting magic, and the strength of nodes close to it. Speculation as to why abounds; and there is even some experimentation going on. Some very, very careful experimentation.
Why careful? Well, the timeline of THE Grid suggests that it existed prior to whatever it was that wiped out the dinosaurs. The mundane world has decided that said wiping out was probably due to an asteroid; mages are not so sure. All they know is that scrying the time period tends to simply bring back a sudden vision of heat, light, and pulverized rock, which could mean anything. Including, say, a central power nexus explosion. Modern mages build modern nexi deliberately small, precisely to avoid a world-spanning catastrophe: but were the mages of THE Grid as wise? Well, nobody’s survived to be asked about that, hey?