What is Distro?
In the Linux world you will hear this odd term Distro, and someone might ask you, "What Linux Distro do you run?", or some variation. Do not take this lightly, this is akin to someone asking your political party, viewpoint, or worse, religion, but you should answer them. People can be very serious about their chosen Linux distro, this can be deeply personal to some, while many may not care what so ever and be agnostic. This is where the Linux world can seem cultish, a bit extreme, or just very different, although, people sleeping outside Apple stores are clearly drinking the cider.
A Linux Distro is just a collection of software put together in some form with a particular package manager and other concepts to be covered, and some nifty name shipped with a Linux kernel, that is all. There are many Linux Distros, Linux Distributions, some are similar, derivatives, or related others completely unique. You can make your own Linux Distro if you so choose! Go you! You may skip ahead to choosing a distro, or continue to following the Tux into Linuxland.
After the Kernel
After the Linux kernel starts, it hands things over to a whole bunch of other software, we won't get into the nitty gritty, because it gets complex and touches on other contested religious matters. The best visual for this is a rubber band ball. You have the Linux kernel at the center, and all this other software wrapping around it in various layers. That is essentially a Linux Distro, a Linux kernel, plus a bunch of other software, some more, others less, and most all highly customizable.
In a Linux Distro, you have a lot more choice and control over what software is installed on your computer, and how that software is configured, and at times, even how it is built. This is a level of user customization and preference that cannot be had with operating systems such as ChromeOS, MacOS, or Windows, including Android and others. Only a few other Unixes like the BSD's and others offer similar, and that is mostly the difference of kernel and some other things, with much of the same software.
Software
Most of the software available on any Linux Distro is Free and Open Source Software (FOSS), and but not all and there are many licenses for FOSS. There are regular closed source free and paid software, along with paid support for FOSS. A lot of this software can run on any operating system, but some of it is specific to Linux and Unix based operating systems. In making a Linux Distro, people choose what software to put together and how, you could envision a retail store with items on a shelf, all the items are available in any store, but arranged differently, checkout process differs, bagging, etc. While there is software specific to various Linux Distros, this is typically in things like the package manager, settings interfaces, and other things that are specific to that distro.
One of the most unique aspect of any Distro is the package manage and system settings, where configuration files are located and how things are managed and configured all around. This is where the differences in Linux Distros mainly reside, software is mostly all the same, but how you obtain, install and configure that software varies. There is also branding and other differences, but in general, that is what makes one Linux distro different from the next.
There are many more details, but there is information all over and this is intended to give you an overview, not in depth details. The next stop on your Linux journey is choosing a distro, which will give you more information on the differences in Linux Distros, package managers, and other things that will help you on your journey.