In common usage, Linux is an operating system built around the Linux kernel. This operating system includes many other components, notably from the GNU project, so the term GNU/Linux is sometimes used. There are many different Linux distributions (see distros), including more or less the same programs but with different installers and other customizations.
Use this tag for questions that are specific to Linux-the-operating-system, such as hardware support, networking setup, filesystems, etc. Most questions should have other tags to identify the area of interest, e.g. sound, console, iptables, proc, partition, etc.
The Linux kernel is used in some non-unix-like embedded operating systems, the best known being Android. These operating systems are off-topic here (you can ask questions about using Android on Android Stack Exchange.