What is a Linux Distribution?
A Linux distribution, also called a distro, is an operating system distributed by someone that uses Linux as its kernel. For example, Ubuntu, Linux Mint, Arch Linux, and Fedora are Linux distributions. Usually this means there will be a website where you can download the distribution from.
Linux, being only the kernel, isn't enough to create a full operating system. Most distributions are GNU/Linux as they include GNU tools such as Bash and glibc in the distribution. One exception being the Alpine Linux distribution. Many distributions also include a daemon called systemd, leading some to jokingly call them systemd/GNU/Linux.
In essence, a distribution is a collection of random pieces of software packaged together with Linux at its core to form a complete operating system. Part of this software are default configurations such as where (which software repositories) the system gets its updates from.
However, a GNU/Linux operating system isn't a graphical operating system like Windows, i.e. there is no start menu, taskbar, and you can't open a window, it's just the terminal. For that, other software is needed, such as a desktop environment, of which there are several, such as KDE, XFCE, GNOME, Cinnamon, etc. In this case, a distribution may refer to them as flavors. For example, Ubuntu comes with GNOME configured by default, while Kubuntu is KDE, and Xubuntu is XFCE.
Some distributions are based on other distributions, e.g. Linux Mint is based on Ubuntu, which in turn is based on Debian. This means they share a lot in common making tutorials for one system usually applicable to similar systems.