The PlayStation Network undergoes maintenance all the time. Usually, though, it only lasts for 2-4 hours. On Thursday, the network is undergoing some serious maintenance, because it'll be down for most of the American day.
Starting at 8am Pacific and running until an estimated 10pm Pacific, almost everything PlayStation-related that uses the internet will be going offline. That means the PlayStation Store, online multiplayer, signing into accounts, editing accounts and PlayStation Home. If you're signed in once the maintenance starts, you'll be booted out.
You won't even be able to visit the PlayStation Blog for updates, because it'll be down as well. In fact about the only thing running will be Sony's PlayStation Twitter account, so maybe you can play with your friends on that...