Apple is the technology giant responsible for the iPhone, the iPod and the iPad. Apple originally came to prominence making PCs and then laptop machines and they still make a range of these devices.
Apple use their own Operating System (OS) developed from the Open Source, Unix like, system BSD (Berkeley Software Distribution). Apple's OS is called OS X (OS 10). Though proprietary and not Free & Open Source (FOS) Mac OS X shares many similarities with Linux and Android and there are many FOS applications available for OS X.
Apple's OS X is arguably the most sophisticated OS available for computer systems and, despite the company's recent emphasis on mobile technologies (such as the iPhone), served by its derivitive OS for mobile devices (iOS), its computers are still pre-eminent in many high-end environments, like music production and the video effects industry. OS X is well supported when it comes to Office and creative applications.
OS X remains one of the simplest and most intuitive systems to use and has the best integration, at the time of writing, with assistive technologies for disabled users. With most software required for such users built-in to the OS out-of-the-box. This is also true of iOS.
As OS X is a Unix-like OS it is much more difficult for criminals to create viruses and Mal-wear for this OS and that, coupled with the fact that OS X is not as prominent as Windows, means that there are, at the time of writing, no such hazards at large to worry users.