Linux is the generic term for a UNIX-like open source operating system based on the Linux kernel. Use this tag for questions about DevOps practices, processes and tools in Linux environments.
GNU/Linux is a UNIX-like Open Source operating system. Linus Torvalds originally wrote the Linux kernel with contributions from others on the MINIX Usenet board. Since then, Linux had thousands of developers, both paid by companies and volunteers. The primary system tools and libraries sourced from GNU Project, hence the nomenclature GNU/Linux (GNU Toolchain over Linux Kernel). The license for the Linux kernel the GPL v2 license.
There are many different Linux Distributions (sometimes shortened to distro), which consist of software applications packaged and provided with the Linux kernel, being Slackware the oldest active distro and Debian, Red Hat and CentOS among the most used on Internet servers¹.
The Linux kernel is commonly run on servers but is highly portable and used in a variety of applications, from wireless routers and cell phones to clusters and supercomputers with thousands of nodes and processors.
For generic Linux questions, consider asking at Unix & Linux Stack Exchange.