Linux refers to both the core kernel used in a huge range of computing devices, from tiny embedded devices to the largest and most powerful mainframe computers available, and a computer operating system (OS) for desktops, laptops, tablets, phones, servers and all kinds of embedded devices. It is libre software meaning its program design (source code) is freely available for anyone to use, copy, modify, sell, rent or publish. This makes Linux fundamentally different from the Microsoft and Apple operating system, which do not allow their users any of these freedoms. Several years ago Linux used to be quite difficult to use but there are now versions of Linux with easy-to-use graphical user interfaces and a huge amount of software ranging from office-productivity software to complex scientific software. Linux uses the ALSA sound system to provide support for multimedia applications.