Linux is the generic name used for a number of distributions (software collections) based on the Linux Operating System (OS) Kernel, originally developed by Linus Torvolds.
The Linux Kernel is a 'Unix type' OS which is distributed under the GNU Free and Open Source (FOS) licence, meaning that it can be used and distributed free of charge but, more importantly, free of restriction - FOS applications, including the Linux Kernel, can be modified by anyone, provided that the source code for the modifications are made available to the FOS community. What this means is that developers are able to take other developers' code and further develop it, then make it available to everyone. This allows software to be developed and distributed free of charge and also updated very quickly.
There are many Linux distributions, some of the biggest and best known are: Ubuntu, Mint, Elementary (all based on the Debian distribution design), Fedora (based on Red Hat), SuSE and Open SuSe, Slackware, Mandreva, Enlightenment and more than a hundred others.
A typical Linux distribution includes a Graphical User Interface (GUI), similar to Android, Windows or Mac OS X, and a selection of FOS applications - there are more than 15,000 FOS applications to choose from and many of these are available, cross-platform, on Android, OS X and Windows. Obviously, Linux is well supported when it comes to Office and creative applications.
For instance; Libre Office is a fully featured Office package, highly compatible with Microsoft Office, with Word-processor, Spreadsheet, Presentation and Database functionality but also including a Vector drawing suite, Mathematical suite and Portable Document Format (PDF) editing facilities built in. It is currently available for Linux, Mac OS X and Windows, with a limited version also available for Android.
Linux distributions, such as Ubuntu and Mint, are very similar in use to Windows, Mac OS X or Android, though the layout of the GUI will be different, e.g. Mint is similar in layout to the old Windows XP OS, Elementary is similar in layout to Mac OS X and Ubuntu is more individual in its design. All, however, offer a genuine and practical alternative to Windows, Mac OS X or Android.
As the Linux Kernel is a Unix-like OS it is much more difficult for criminals to create viruses and Mal-wear for this OS and that, coupled with the fact that Linux is not as prominent as Windows, means that there are, at the time of writing, no such hazards at large to worry users.