A quick press of two keys triggers the visualizer: while listening to music, just press Command-T, and sit back and watch the show.
As you can see in the screenshot above, the visualizer displays the name of the track and album, its artist, and its artwork, at the bottom left corner of the window for a few seconds. That way, you can always glance at your display when a new track starts playing to see what it is.
If you want the visualizer to fill your display, choose View > Enter Full Screen, or press Control-Command-F. When you’ve had enough of your show, press Command-T to stop it.
If that’s all the visualizer did, it would be pretty cool, but it gets a lot better. If you choose View > Visualizer, you’ll see that you can choose from several different plug-ins. There’s also a Play Videos setting. Select this if you’re playing music from a playlist that contains music videos, and you want iTunes to play the videos when it gets to them, rather than use the visualizer.
The more changes there are in the music, the livelier the visualizer will be. You’ll see that percussive songs lead to a lot of effects, but the visualizer also adapts to mellow music to provide a constantly changing palette of colors and shapes. (To be fair, I find it a bit too busy for ambient music, where the lights should be more relaxing. But read on to learn how you can change that.)
Try with different songs and see how the visualizer changes. By default, it uses a random setting that cycles through a number of presets. But you can change the visualizer’s settings too. Press to see your options. Here are the options for the iTunes visualizer.
The following keys aren’t shown on the Help screen:
Note that you can pause and resume playback while the visualizer is active by pressing the spacebar, and you can skip tracks by pressing the right-arrow (next) or left-arrow (back) keys.
You can choose from other visualizers by selecting one in the View > Visualizer menu. The settings available for each visualizer are different. For the iTunes Classic Visualizer, see this article. It’s a lot less interesting than the newer iTunes visualizer.
You can make lots of interesting desktop backgrounds using the visualizer. Just press Command-Shift-3 to take a picture of the screen at any time; it will be saved on the desktop. Open System Preferences, click Desktop & Screen Saver, then Desktop. Drag an image to the well near the top of the window to use it as your wallpaper.