That’s because iTunes is much more than a companion to the iPod, much more than a media playback program and even more than a front door to Apple’s online download service. It’s a sort of miniplatform hiding right within Windows that allows Apple and other companies to connect a host of hardware and software, and to create media-sharing networks without engaging with Windows itself or with Microsoft’s built-in Windows Media Player.

There are way more copies of iTunes installed than were bundled with Apple’s 100 million or so iPods. In fact, at last week’s D: All Things Digital conference, Apple CEO Steve Jobs estimated more than 300 million copies of iTunes were installed. Many people download and use iTunes to play music and video, and to purchase media, for use on their computers, even if they don’t own iPods.

And the vast majority of iTunes installations, probably 90% or more, are on Windows computers, not Macs. Ironically, that makes Apple, Microsoft’s ancient rival, one of the biggest software developers for Windows.

Many people don’t realize that every time they install iTunes on a Windows PC, they also are installing Apple networking software called Bonjour, which operates independently from the Microsoft built-in network software controlled from the Windows Control Panel. This Apple network layer isn’t harmful and doesn’t interfere with the Microsoft networking functions. It’s designed to allow iTunes users to share their music.

Out of the box, each copy of iTunes looks for other shared iTunes music libraries on your local network. It doesn’t share your library unless you authorize it to do so. The user merely has to go into iTunes’ Preferences function (under the Edit menu in the Windows version), click on the Sharing tab and select “Share my library on my local network.” You can choose to share your entire library or just selected playlists. You can require people to enter a password to gain access, or not. You can also turn off the function that allows you to see others’ libraries.

If you use Sharing, you’ll see in iTunes’ left-hand panel a list of shared libraries on other iTunes-equipped computers on your local network, whether they reside on Windows or Macintosh computers. Clicking on these libraries allows you to play the songs they contain. It doesn’t allow you to transfer the song files among the computers.

In many homes, offices and college dorms, iTunes users have access to numerous libraries on nearby computers. For instance, as I write this on a Mac laptop in my home office, I am playing a song that resides on a Windows Vista desktop PC in another room. To achieve this feat, I didn’t have to fiddle with the often confusing network settings in the Windows Control Panel or in the Mac’s similar System Preferences program. I just had to use iTunes on both machines and click a couple of buttons.

In effect, each copy of iTunes, with the user’s permission, broadcasts a sort of beacon that signals its presence to other copies of iTunes on a local network, regardless of the operating system underneath. It makes the operating system irrelevant.

This independent iTunes networking capability goes way beyond sharing music among computers. A modified version of this function is what allows Apple’s new Apple TV product to fetch all the music and videos from all the computers in your house and play them back through your TV set — even if all those computers are Windows machines. It could also allow Apple’s forthcoming iPhone to wirelessly stream music and videos from computers on a local network, if Apple chose to build in such a function.

And the use of iTunes as a platform goes even beyond this networking ability. Small companies have released a slew of programs, such as iLike, last.FM, Mog and Nutsie, that read the iTunes library, with your permission. They use this information to determine your musical tastes and suggest new songs to try, to connect people with similar tastes, or to allow you to listen to your songs over the Internet.

Microsoft, its hardware partners and other third-party companies have achieved similar feats with other music programs and with hardware such as Xbox game consoles using similar sharing technologies. But the popularity of iTunes, and Apple’s position as Microsoft’s rival, makes the iTunes platform far more significant — and interesting.

• Email me at mossberg@wsj.com. Find all my columns and videos online free at the new All Things Digital Web site, http://walt.allthingsd.com.

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