Склонная Ubuntu - получает обновление по сравнению с обновлением

Simply:

apt-get update will update your local copies of your repositories' package data, such as available versions and dependencies.

• This is needed to check whether any updates are present.
• It doesn't actually upgrade packages.

apt-get upgrade and apt-get dist-upgrade will upgrade packages.

• The former runs general system upgrades
• The latter will apply higher level patches such as kernel upgrades.
• Check this link for a detailed look into upgrade vs dist-upgrade

For a detailed discussion of aptitude vs apt-get, read this AskUbuntu post.