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The kernel makes its services available to the application programs that run on it through a large collection of entry points, known technically as system calls.

From a programmer's viewpoint, these look just like ordinary function calls, although in reality a system call involves a distinct switch in the operating mode of the processor from user space to kernel space. Together, the repertoire of system calls provides a 'Linux virtual machine', which can be thought of as an abstraction of the underlying hardware.

An even less visible function of the kernel, even to programmers, is memory management. Each process runs under the illusion that it has an address space (a valid range of memory addresses) to call its own.

: it’s about mirrorin

According to Wikipedia, “In computing, a mirror is an exact copy of a data set. On the Internet, a mirror site is an exact copy of another Internet site.” When you try to install a new package into your Fedora installation, either using PackageKit or Yum, it tries to fetch the packages from an Internet site along with the libraries and other software required for it, and install it on your computer. Now software like OpenOffice.org or OpenArena are very big and along with all their dependencies, the download size may be in the order of hundreds of megabytes.