Linux

Easily the most influential Operating System in the world today, Linux powers the majority of computing devices currently in use. Most obvious to users, Linux forms the foundation for the Android Operating System, which today controls more than 70% of the global smartphone market. Linux is also used in Chromebooks, Linux Desktop computers (which aren't very common, and are mostly custom installations), and nearly all smart appliances. Many networking tools run Linux as well, though some also run FreeBSD Operating Systems, which aren't connected to Linux officially, but are often closely tied from a software standpoint. 

Linux is known for being extremely customizable, as all of its essential components are free and open-source, meaning taht anyone can write code that interacts with the Operating System in any way they wish. Furthermore, anyone can create a spin of Linux using the original code, if they so desired. Because of this, Linux has an extremely broad ecosystem of development tools available to it. However, many of these tools are not well-known, as they are primarily used within the computer industry. Accordingly, many well-known public-facing tools, such as Microsoft Office, are not available for Linux without the use of emulators or compatibility layers. 

So, what are the main uses of Linux? 

Linux is capable of being a very lightweight Operating System, which makes it useful in situations where a full computer is needed, but the computer in question need not be powerful at all. Rather than installing a heavy, desktop-focused Operating System like Windows, many manufacturers opt to create their own custom version of Linux, and deploy that to their smaller machines. 

When smartphones entered the public market, Linux was a natural choice as a foundation for a new mobile Operating System. Smartphones needed to be much more efficient than most user-facing computers, and needed to generate much less heat, as a smaller device's cooling capabilities are much more limited than those of a larger device. As a result, smartphone manufacturers decided to use ARM-based processors, as the ARM instruction set requires less power, and excels at performing its set tasks efficiently. One weakness of ARM is that it can be more difficult to switch out permanent hardware components like Drives and RAM, making it an undesireable choice for user-facing computers at the time. However, the RAM and storage in a smartphone didn't necessarily need to be replaced, as the device was so small that to do so wouldn't be practical anyway, and so ARM was a good option. However, because ARM wasn't deployed to user-facing computers very often, the Windows Operating System did not support it, so Google came up with their own new Operating System designed for smartphones, which was based on Linux, which already had the necessary support. This new Operating System was called Android, and today is the dominant mobile Operating System in the world, controlling over 70% of the global market.