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На официальной странице man apt-get (короткая версия):

   clean
       clean clears out the local repository of retrieved package files.
       It removes everything but the lock file from
       /var/cache/apt/archives/ and /var/cache/apt/archives/partial/. 
       This frees up disk space

   autoremove
       autoremove is used to remove packages that were automatically
       installed to satisfy dependencies for some package and that are no
       more needed.

Существует также gui computer-janitor (System => Administration => System Janitor)