What's this 1394 network adapter on my machine?

IEEE 1394 is the industry standard for a an external data connection also known as "Firewire" (trademarked by Apple), i.Link and a few other marketing monikers.

They all boil down to the same thing: a high speed external interface for data transfer.

Since you're seeing 1394 in your network connections, chances are you have one or more 1394 ports on your machine. They're easy to identify by their distinctively shaped socket. Here's a picture of the side of my MacBook Pro that shows the three common interfaces side-by-side:

Naturally the placement on your own machine might be quite different.

USB and 1394 are sometimes considered "rival" interfaces, since they do many of the same things. 1394 has a slight advantage in popularity for streaming media - if your video camera has a digital interface, chances are it's 1394. USB has an advantage for random peripherals like mice, keyboards and the ever popular USB FlashDrive. And both 1394 and USB are often used for external hard disk drives.

So why does 1394 show up as a networking interface?

It is possible to use a 1394 interface for limited networking. You can, for example, use 1394 to create machine-to-machine network. Naturally, both machines must have the 1394 hardware interface and the appropriate 1394 cable must be used to connect them. Much like networking with a single ethernet cable, I'd expect additional protocols might be required, or additional configuration steps needed in the TCP/IP configuration (since IP addresses would have to be assigned manually.)

But, in all honesty, that's not what 1394 was really designed for. While it might be possible, I wouldn't recommend it, as you won't find a lot of how-to information.

And you certainly cannot use your IEEE 1394 interface to connect to an existing ethernet based network. At least, not without some kind of adapter hardware, and I'm not sure that hardware even exists.

And lastly, rumor has it that 1394 networking ability has been removed from Windows Vista.

My recommendation is to use the interfaces for what they're best at and most commonly used for: USB for peripherals, 1394 for video cameras and perhaps external hard drives, and ethernet for networking. You'll get the best results, and the best support for your efforts, by following those common paths.