A distribution is a kernel plus a userland plus three decisions: a package format, a release policy and a support promise. The kernel is close to identical everywhere. The decisions are what you feel daily.
Nearly every server you will meet descends from one of two ancestors. The Debian 319 family uses .deb packages, dpkg underneath and apt on top; Ubuntu 225 is a Debian derivative, and most desktop Linux is an Ubuntu derivative. The Red Hat family uses .rpm, rpm and dnf; Fedora 1,480 feeds CentOS 2,757 Stream, which feeds Red Hat Enterprise Linux 664 , which is rebuilt by Rocky Linux 11,029 and AlmaLinux 7,122 . The rest are a rounding error in web hosting, except Alpine, which you meet inside Docker 514 images because it is small.
The family decides which half of the internet's advice applies to the machine in front of you.
| Detail | Debian and Ubuntu | RHEL, Rocky and AlmaLinux |
|---|---|---|
| Package tools | apt, dpkg (.deb) | dnf, rpm (.rpm) |
| Apache 129 package | apache2 | httpd |
| Apache config root | /etc/apache2, a2ensite | /etc/httpd/conf.d |
| Web server user | www-data | apache |
| PHP under Apache | mod_php or PHP-FPM | PHP-FPM only |
| Host firewall | ufw 280 | firewalld 292,900 |
| Access control layer | AppArmor 454,622 | SELinux 1,634 |
Running the wrong family's commands fails loudly and harmlessly — apt does not exist on Rocky Linux. The dangerous case is the config path: editing /etc/httpd/conf.d on Ubuntu creates a directory Apache never reads, so the server keeps serving the old configuration while you wonder why nothing changed.