As illustrated above, the Linux kernel is the core of a computer operating system. A full Linux distribution consists of the kernel plus a number of other software tools for file-related operations, user management, and software package management. Each of these tools provides a small part of the complete system. Each tool is often its own separate project, with its own developers working to perfect that piece of the system.
As mentioned earlier, the current Linux kernel, along with past Linux kernels (as well as earlier release versions) can be found at the www.kernel.org website. The various Linux distributions may be based on different kernel versions. For example, the very popular RHEL 6 distribution is based on the 2.6.32 version of the Linux kernel, which is rather old but extremely stable. Other distributions may move more quickly in adopting the latest kernel releases. It is important to note that the kernel is not an all or nothing proposition, for example, RHEL 6 has incorporated many of the more recent kernel improvements into their version of 2.6.32.
Examples of other essential tools and ingredients provided by distributions include the C/C++ compiler, the gdb debugger, the core system libraries applications need to link with in order to run, the low-level interface for drawing graphics on the screen as well as the higher-level desktop environment, and the system for installing and updating the various components including the kernel itself.
A vast variety of Linux distributions cater to different audiences and organizations depending on their specific needs. Large commercial organizations tend to favor the commercially supported distributions from Red Hat, SUSE and Canonical(Ubuntu).
CentOS is a popular free alternative to Red Hat Enterprise Linux (RHEL). Ubuntu and Fedora are popular in the educational realm. Scientific Linux is favored by the scientific research community for its compatibility with scientific and mathematical software packages. Both CentOS and Scientific Linux are binary-compatible with RHEL; i.e., binary software packages in most cases will install properly across the distributions.
Many commercial distributors, including Red Hat, Ubuntu, SUSE, and Oracle, provide long-term fee-based support for their distributions, as well as hardware and software certification. All major distributors provide update services for keeping your system primed with the latest security and bug fixes, and performance enhancements, as well as provide online support resources.