If you play World of Warcraft, or still play Starcraft or Diablo online, don't watch this. You'll learn nothing. But for the hordes of people signing up to Blizzard's online service for the first time today, this is most helpful.
It's a beginner's guide to Battle.net, the online authentication, matchmaking and social service that runs (and protects) Blizzard's games for the PC and Mac.
Like I said, for experienced players this will be sleep-inducing, but for the uninitiated - which was me until a few months back - this will tell you everything you need to know about Blizzard's annoying (if also slightly helpful) front end.