pacman and the AUR (Arch)

In short: pacman is the fast package manager of Arch Linux. The AUR (Arch User Repository) adds user packages that you build yourself from build recipes (PKGBUILD).

In more detail: Arch is a rolling release: there are no versions, just constantly current packages. Official packages come from the repositories core, extra and multilib.

In detail

sudo pacman -Syu             # update the system (always completely)
sudo pacman -S vlc           # install
sudo pacman -R vlc           # remove
sudo pacman -Rns vlc         # including dependencies and config
pacman -Ss word              # search
pacman -Qi vlc               # info on an installed package
pacman -Qdt                  # orphaned packages

• Avoid partial updates: pacman -Sy package without -u can break the system.
• AUR: packages from the AUR are fetched with a helper such as yay or paru. Read every PKGBUILD first, because user packages are not vetted.
• Configuration: /etc/pacman.conf, mirrors in /etc/pacman.d/mirrorlist.
• News: before big updates read the notices on archlinux.org.
• .pacnew files: compare new configuration templates after updates with pacdiff.

See also: Arch Linux, Package management on Linux at a glance