Apple had three favorite typefaces: Myriad, Lucida Grande and Helvetica Neue. Now there's a new favorite: Avenir. It has appeared simultaneously OS X Mountain Lion and iOS 6—which means you will see it featured in the next iPhone.
All these sans serif typefaces have a few things in common, but they all have a very distinctive look and role.
Back in 2002, Apple adopted Myriad as its corporate typeface. It replaced Apple Garamond, Cupertino's own version of the classic Garamond, a serif typeface that was everywhere—from the company logo to all of its computers, including every single model of Macintosh and PowerBook released until the eMac. It was so iconic that long-time Apple users protested when the company abandoned it.
Myriad is an Adobe typeface, created by Robert Slimbach and Carol Twombly. You can find it in every single Apple product box, advertising and marketing material, even its own logotype. Some fonts of the Myriad typeface are included in OS X, but it is completely absent from fonts included with iOS.
Despite its prominence in Apple's corporate image, Myriad is not the typeface of choice for its iDevice or desktop operating systems. For the latter, Lucida Grande gets the nod, a sans serif that became the system font for OS X. It was designed by Charles Bigelow and Kris Holmes to replace Charcoal, the sans-serif system font for Mac OS 8 and Mac OS 9 that took over another old timer: Chicago. The latter was a much-beloved screen font designed by Susan Kare for the original Macintosh operating system.
Like Myriad, Lucida Grande is not included in iOS.
The third typeface is Helvetica Neue, which replaced Helvetica in iOS 4. Helvetica appeared for the first time in an Apple product on the iPhone 3G and the iPod touch. It was also a replacement for another typeface, called Podium Sans, which was the standard typeface for both the original iPhone and most of the iPods. The original iPod didn't include Podium Sans, but the good old Chicago.
Helvetica and Helvetica Neue are also included in OS X.
Update: In fact, the latest update of iPhoto is using Helvetica Neue.
And now there's Avenir, a Linotype typeface and Apple's new favorite. It was designed by Adrian Frutiger in 1988, inspired by two old school typefaces created in the 20s: Erbar by Jakob Erbar and Futura by Paul Renner.
It has acquired a special role in iOS because is the typeface of choice for two of the most important—and perhaps most crucial—new features in iOS 6. First is the new Apple-created Maps, which is one of Apple's big gambles in some time. It's also included in the new screens of Siri. You can find it featured prominently in the restaurant recommendation page, for example.
Avenir is included in OS X too. In fact, you can find three typefaces of the family (Avenir, Avenir Next, and Avenir Next Condensed), all in a variety of weights. Apple is not using it in any OS X application (or at least I can't find it anywhere).
Would Avenir take an special role in OS X too? Maybe. Perhaps Apple will unify all of its system typefaces one day, if iOS and OS X finally merge into a single operating system. And perhaps that day everything will be Avenir—I hope not, since Helvetica Neue is much prettier and more legible. Update: Since iPhoto uses Helvetica Neue after the latest Mountain Lion update, maybe that would end being the system typeface of the future OS X/iOS.
And if you're wondering why it matters when a company chooses one typeface to replace another, just remember: it could have been Comic Sans.