Ubuntu apt update vs upgrade vs dist-upgrade vs do-release-upgrade

• apt update – gets an up to date list of packages that can be installed or updated
• apt upgrade – updates packages it can, but never removes packages
• apt full-upgrade – updates packages more aggressively, removing packages that block other updates
• apt-get dist-upgrade an old name for full-upgrade
• do-release-upgrade upgrades to the latest operating system version

These are all confusingly named, and could have been called sync, safe-update, force-update, and upgrade-os.

See also: man apt