Linux is a free open-source operating system based on Unix. Linus Torvalds originally created Linux with the assistance of developers from around the world. Linux is:

    Unix Like
    Open Source
    Network operating system

Strictly speaking, Linux is a kernel. A kernel provides access to the computer hardware and control access to resources such as:

    Files and data.
    Running programs.
    Loading programs into memory.
    Security and firewall.
    Other resources etc.

The kernel decides who will use a resource, for how long and when.You can download the Linux kernel from the official web site. However, the Linux kernel itself is useless unless you get all the applications such as text editors, email clients, browsers, office applications, etc. Therefore, someone came up with idea of a Linux distribution. A typical Linux distribution includes:

    Linux kernel.
    GNU application utilities such as text editors, browsers etc.
    Collection of various GUI (X windows) applications and utilities.
    Office application software.
    Software development tools and compilers.
    Thousands of ready to use application software packages.
    Linux Installation programs/scripts.
    Linux post installation management tools daily work such as adding users, installing applications, etc.
    And, a Shell to glue everything together.