There are a few terms that may confuse Linux beginners. The first thing is its name, Linux vs GNU/Linux, the term Linux refers to the Linux kernel only. In reality many users refer to Linux as the operating system as a whole, the kernel plus libraries and tools. Also the term Linux is used to include all the programs that run on Linux, or that are available for this great operating system.
Furthermore, the description GNU/Linux needs understanding. Linux distributions with this name prefix are fleshed out with GNU implementations of the system tools and programs. One such example is Debian GNU/Linux. The GNU project goes back to the initiative of Richard M. Stallman and his dream to develop a free UNIX system. Based on his experiences at MIT and the collaboration with other colleagues he choose to use free software that was already available to rewrite the tools he needed. This included the TeX typesetting system as well as X11 window system. He published the rewritten tools under the GPL license whenever possible to make his work available freely to everyone who was interested in it.
A Linux distribution is a collection of software packages that fit together. A distribution is maintained by a team of software developers. Each member of the team focuses on a different package of the distribution. Together as a team they ensure that the single software packages are up-to-date and do not conflict with the other packages of the same release of the distribution.
As of 2019 for Debian GNU/Linux 10, the distribution includes over 13,370 new packages, for a total of over 57,703 packages. A repository is a directory of packages with a certain purpose. Debian GNU/Linux sorts its packages according to the development state. The official repository is named stable and reflects the current release of stable packages. The other repositories are named testing and unstable, and work in the same way but do not count as official packages.
Typically a Linux distribution comprises of packages for a Linux kernel, a boot loader, GNU tools and libraries, a graphical desktop environment with a windows environment, as well as additional software like a web browser, an email client, databases and documentation. The software is provided in two ways; as the source code and as the compiled binary packages. This allows you to understand how the software is designed, to study it and to adjust it according to your personal needs.
Depending on the focus of the Linux distribution, it also contains packages for a specific purpose like network or forensic tools, scientific software for educational purposes, and multimedia applications.