Updated from 10:31 a.m. to include more information in the fifth paragraph.
Healthbook, similar to Passbook, is an app that Apple is building on that will store all of your health and fitness related data in one place, as Apple makes its move into wearable technology, and looks to the health and fitness market as its next big opportunity.
Of the more notable features included in Healthbook (Gurman cautions that this could be for Apple's next mobile operating system, iOS 8, but could be pushed back or canceled entirely), it tracks steps taken and calories, as well as monitoring things like blood pressure, oxygen levels, sleep activity and even monitors your breathing. There are several other features included in the app, but these are some of the most important.
Whether Healthbook is released as part of iOS 8 or as part of a new hardware (iWatch) remains to be seen, but it's obvious that the company is moving into the health and fitness market, which could be a huge market for Apple, according to Morgan Stanley analyst Katy Huberty. "Our working assumption is that iWatch will largely be adopted as an accessory device and therefore sold into the existing customer base like the iPad rather than to new customers like the iPod or iPhone," Huberty wrote in Feb. research note about Apple. "Assuming an ASP of $299 and Apple customer base penetration rate similar to the iPad, we see up to $17.5B of revenue in the first 12 months compared to $12B for the iPad and $2.5B for the iPhone."