Linux is an open source operating system. Like other operating system (such as Windows) Linux consists of various software components that manage computer hardware resources and enable you to do tasks such as surfing the web or editing a file in a text editor.
Linux is a free and open source software, which means that you can use, copy, study, and change the software in any way. It is distributed with the source code so users can view and modify it. This is in contrast to Microsoft Windows, a proprietary operating system.
Many versions of Linux exist. Some of the more popular Linux distributions are Debian, Ubuntu, Fedora, openSUSE, Red Hat Enterprise Linux and SUSE Linux Enterprise Server.
Today, Linux runs almost every computing platform available: mobile phones, tablets, servers, desktops, mainframes, etc. Android, a widely used operating system for mobile phones and tablets is based on the Linux kernel.
Modern Linux distributions usually feature a nice GUI, similar to the one found in Windows. Here is an example Linux desktop in SUSE: