Distribution (distro)
The kernel plus everything else, packaged as a usable system.
Linux itself is only the kernel. A distribution bundles it with GNU tools, a package manager, an init system, drivers and defaults to make a usable operating system: Ubuntu, Debian, Fedora, Arch, Alpine.
They differ in package manager, release cadence and defaults, but the commands in this reference behave the same across all of them. That portability is exactly why terminal skills outlive any particular distro choice.
See it in practice
$ cat /etc/os-releaseNames the distribution and version. The first command to run on any machine you did not set up yourself.
Where you'll meet it
Search, install, upgrade and remove packages, plus the dnf/pacman equivalents.
KernelThe core of the operating system, managing hardware and processes.
Package managerThe tool that installs, updates and removes software.