apt-get update : is use for updating all your current catalog for the lastet update
apt-get upgrade : is use to upgrade apps that u have install on your N900
apt-get upgrade -a : is use to upgrade all apps without having to choose which one you want to upgrade
apt-get remove : is to remove apps from your N900
apt-get remove --purge : is use to fully remove apps & it setting from your N900
apt-get autoremove : is use to autoremove files that have been left by normal uninstaller
apt-get autoclean : is to clean your N900 from temp flies n etc
apt-get -f install : is use to fixed some installer that is not fully configurate by your installer