The term distribution refers to a complete GNU/Linux operating system. Commonly shortened to just "distro", a distribution includes the Linux kernel, hundreds of small GNU programs and tools, and usually a windows manager or three. The term Linux technically only refers to the kernel of the OS, though the generic term Linux is often applied to the entire distro.
Examples of common distributions today are Red Hat, Ubuntu, Gentoo, and Slackware. While each of these distributions contain the same basic Linux kernel (or a variation of it) and the same basic tools, they all represent different philosophies and goals. Red Hat is a stable, supported distro that large companies pay for. It is usually maintained by a professional systems administrator or IT department. Ubuntu, on the other hand, is available free of charge, and support is provided by the Ubuntu community. Optionally, Ubuntu users may purchase a support contract, that assumes a far lower level of user competence than does Red Hat. This is because Ubuntu is not aimed at large corporations, rather, at the average home user. Gentoo and Slackware are also aimed at home users, but assume a greater understanding of CS concepts than do Ubuntu. Most users of Gentoo and Slackware have degrees in computer engineering, and work in high-tech fields. Despite this, Gentoo and Slackware are fundamentally different in the way the user maintains and updates the distro.
Linux distributions were invented because the Linux kernel is developed separately from the common tools and programs that are usually associated with it. The simplest of operations, such as copying files, are done by programs outside the kernel, and the kernel is basically useless without them. Bundling them together into a distribution creates an entire operating system that is capable of performing all the functions expected of an OS. Most Linux distributions available today contain enough tools and programs to be complete unto themselves, without the user needing to acquire third-party software to browse the web, read email, open office documents, and most other common activities. The oldest Linux distribution still in use today is Slackware.
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