To answer this question one must split it. Plus make other questions like: «What is a, so called, Linux distro?». It seems complex and it is in a way. But let´s explain GNU/Linux in simple terms.
GNU stands for GNU is Not UNIX. So it´s clear is not UNIX but it´s related to it in some fashion. It was started in 1983 by Richard Stallman who at the time had just left the MIT (Massachussets Institute of Technology). He, as a computer scientist, had been working in the field for some time and saw a problem using the systems and the change of the mood at the time. Software had been shared more or less freely as well as ideas, although some works were copyrighted and some licenses applied. Since the value of computers was starting to be seen as very profitable companies and institutions began to put restrictions on code sharing, accessibility, licensing and user agreement terms, etc. Mr Stallman was unhappy about this. So he started his own project.
Countering the proprietary concept Mr Stallman not only started the GNU project but created a new licensing model based on some good moral principles which are heavily standing on freedom. To avoid the turmoil of releasing the code of a new operating system without licensing just as a public domain work, he flipped over the copyright concept. Copyleft was the new word. The basic idea is the work has an owner just as in the copyright frame but it can be freely distributed, adapted and even changed, provided the same licensing terms are kept. This very concept was the pillar on which the GPL license grew. In short the licensing terms are based on the following principles:
Freedom 0 – the freedom to use the work,
Freedom 1- the freedom to study the work,
Freedom 2 – the freedom to copy and share the work with others,
Freedom 3 – the freedom to modify the work, and the freedom to distribute modified and therefore derivative works.
Not only Mr Stallman was unhappy about the proprietary terms he had to deal with, some others were as well. So a community formed around the GNU project. The aim was to publish an operating system similar to UNIX in concepts and forms but not bound to a copyright restrictive license model but to a more freely copyleft one.
Building an operating system is not an easy task. And it´s even a bit more difficult if you are not a company who sells it. But let´s put the economical questions aside. An operating system has different components. Its mission is allowing users run their programs without a hassle, providing the necessary services, stability, reliability and predictability to do so. The OS is composed by programs which accomplish specific tasks. Programs that talk to other programs. And so on and so forth. One key part of an operating system is the kernel. A kernel is the program that allocates hardware resources to the rest of the components. The GNU project was developing most of the components of an operating system. Years went by and one component was missing, the kernel. The model they had chosen was a microkernel instead of a more mainstream (but less atractive) one like the monolithic kernel found in, for example, the Minix operating system or other proprietary UNIX systems.
The GNU operating system was not the only project releasing code under open terms. At the Berkeley university there was another project building UNIX. The university had set a team of individuals, teachers and students, called the Computer Systems Research Group. They had access to original UNIX code from Bell Labs and AT&T. They were extending the codebase and they were sharing their code to other universities as well as to the UNIX parent companies. For example the vi editor and the whole network stack found in UNIX came from the contributions of this group. This form of UNIX was known as the Berkeley Software Distribution. At one time, in the beginning of the nineties, much of the original UNIX code contained in BSD had been replaced by home grown work. A company founded by contributors was selling this code. Unix System Laboratories, an AT&T subsidiary, filed a lawsuit against not only BSDi (the company selling this form of UNIX) but to the Computer Systems Research Group from Berkeley University as well, claiming intellectual property over UNIX code. Long story made short: After two years and a change of court allowed the BSD projects to continue releasing code provided they avoided a few files which still had some AT&T original code, while USL agreed not to claim such property rights since they had missed to include CSRG copyrights in the code they were selling. An undisclosed agreement settled the conflict down.
At the same time this was happening a computer science student in Finland, Linus Torvalds, started his little project to recreate some parts of an operating system. Inspired by the SUN OS (a BSD derivative at the time) he had access at the university he started to write code and share it with his fellows. His code was good and got some traction. This took some time obviously but since the GNU project still lacked of a working kernel the marriage was inevitable. With the future of the BSD´s unclear due to the lawsuit all was set for a form of a freely distributable code to florish. Enter GNU/Linux.
Nowadays a distro is a form of a GNU/Linux combination. And it´s most commonly called «a Linux distro» to Mr Stallman´s desperation. It is an operating system composed by different GNU utilities and the Linux kernel. Often times they distinguish themselves by their targeted audience. Desktop oriented, server oriented, embedded, old hardware, appliances, etc. Some companies have developed their own ones being the most relevant nowadays Ubuntu from Canonical Ltd, openSuSE from Novell and Fedora and CentOs from Red Hat. Some others are community driven projects such as Debian, Arch, Gentoo and one of the oldest distros, Slackware.
The differences between the distros can be minimal sometimes but some others they are pretty relevant. Not only for their targeted audience but for the inclusion of different features or program versions. For example Oracle´s Linux is based on Red Hat Linux but it also includes D-Trace which is a tool not commonly found in Linux distros which allows fine grained process inspection. Different kernel versions and differently compiled forms are a typical difference but schedulers, libraries and some other programs are the rest of the lot.
The GNU/Linux success wouldn´t have happened without the blessing of computer giants like IBM. After the UNIX wars, which were the commercial wars occurred during the eighties and part of the nineties, the copyleft nature of GNU/Linux was seen as an advantage rather than a problem. Companies soon realised they had the opportunity to invest little on the parts they were mostly interested while getting the benefits of other´s work. A common ground was found to built their own products on top without the harm of commercial wars of proprietary software. This is good, but it´s also bad. Read the politics section of this site and you will find out more about this.
The promiscuous nature of GPL licensed code has let GNU and Linux, jointly but also separately to spread around the globe into different markets and uses. A good example is the Android operating system found on mobile phones. Linux is the kernel found on those devices. Many servers used for multiple purposes run a GNU/Linux distribution, some commercial ones, others community-driven. It has even found its way into the IBM Mainframe where the three players are SUSE, Red Hat and Ubuntu.
The BSD´s are living a second rebirth so to speak. They never went away and it looks like they never will. Powering Apple products, the Playstation, network appliances from Juniper Networks, the Netflix video stream (the 25% of internet traffic in the USA at times), Whatsapp, Yahoo and Yandex are some of the modern uses of BSD based operating systems.
There are many bits from GNU/Linux left out of this article. You can read many interesting articles on the internet. Go look for them and share your views and discoveries if you wish to.