A complete OS built around the Linux kernel — Ubuntu, RHEL, Debian, Fedora — each bundling its own package manager and defaults.
A distribution ("distro") takes the bare Linux kernel and turns it into something you can actually install and use: a package manager, an init system, default configuration, a set of pre-installed utilities, and — critically — an opinion about how updates and releases should work.
Almost every distro traces back to one of a few lineages. Debian's lineage (Debian, Ubuntu, Mint, Raspberry Pi OS) uses .deb packages and apt. Red Hat's lineage (RHEL, Fedora, CentOS, Rocky, Alma) uses .rpm packages and dnf/yum. A smaller set of independents (Arch, Alpine, NixOS) do their own thing entirely.
The lineage determines almost everything you'll type day to day: install commands, config file locations for some services, default shell, and even how aggressively packages get updated. RHEL and its downstream clones favor long support windows and conservative package versions (good for production stability); Fedora and Arch favor newer packages and shorter support windows (good for staying current).
"LTS" (Long Term Support) releases — Ubuntu 22.04 LTS, for example — get 5 years of security patches and are the default choice for anything running in production, versus non-LTS releases meant for testing newer features.
apt install commands silently fail ("command not found") on RHEL-based systems that need dnf.