I think I can understand your confusion because people use the word 'Linux' to refer to several related things. A very simplified description that is 'right enough' but not completely right in several or many technical details:
'Linux' can refer to the Linux kernel. The Linux kernel is a highly technical computer program that runs as a computer operating system. When a computer is turned on, the Linux kernel starts up and waits for other programs to ask it to do work such as connect to a network or read a file from a disk.
'Linux' can refer to the Linux kernel plus a mostly standard set of programs for interacting with the kernel. The mostly standard set of programs often come from the GNU project and replicate tools that were (and are) commonly found on UNIX computers. These include big tools like the text editors Vim and Emacs; shells like Bash; and common shell programs like
The bundling some programs with the Linux kernel is called a 'Linux distribution' or 'distro'. Ubuntu 16.04, Ubuntu 15.10, Kbuntu 16.04, etc. are different Linux distributions and each bundles a different set of programs with the Linux Kernel. It is probably helpful to think of Ubuntu as a family of Linux distributions. Fedora, Mint, Arch, and Debian are examples of other families of Linux distributions (Ubuntu is a family within the Debian family).
To a first approximation any Linux program will run on any distribution. Often a particular program will be available as a binary executable using the package manager (
apt for Ubuntu and other members of the Debian family). The package manager figures out what other programs are required to run the program being downloaded, which of those dependencies are on the computer already, and retrieves any that are not.
Even when no binary executable is available, the source code usually is and the program can be compiled by the user's computer to create a binary executable file. Compiling from source usually requires opening a command shell and running a script or running
make. These days, that doesn't count as being user friendly and it can take a while. On the other hand, compiling from source makes a vast number and diversity of computer programs available on wide variety of computers.
Ubuntu and other Linux distributions are not as easy to use as the operating system on a smartphone. However, the basic ways in which a person can control Linux are roughly the same as they were twenty five years ago and fairly similar to the ways in which Unix systems could be controlled forty years ago.
So while it takes a relatively long time to work my way through the confusion, almost everything I learn about Ubuntu and Linux is not going to become obsolete when the next version comes out.