On Apple's homepage and other web pages on the site, the San Francisco typeface is being used in many places where text is displayed, replacing the previous Myriad typeface. Bolder and easier to read, San Francisco has been used on iOS devices and Macs since iOS 9 and OS X 10.11 were introduced in 2015.
San Francisco is a condensed sans-serif that's similar to Helvetica. It was created specifically for small displays like the Apple Watch, with extra spacing between letters to increase legibility. It also works well on larger Retina displays because of its clean design.
San Francisco is the first font Apple has designed in-house in many years. In the 80s and 90s, Apple used several fonts that were created in-house, but the company largely stopped making its own fonts in the early 1990s.
Apple isn't yet using the San Francisco typeface for its entire website, but may continue the transition over the coming days.