The illusion is one of Apple’s latest innovations: the Taptic Engine. Relying on a technique pioneered in research labs 20 years ago, it uses an electromagnetic motor to trick your fingers into feeling things that aren’t actually there. The motor’s precisely tuned oscillation makes it feel like you’re depressing a mechanical button, when you’re really just mashing your finger against a stationary piece of glass.

http://www.wired.com/2015/03/apples-haptic-tech-makes-way-tomorrows-touchable-uis/