The terminal, the console the command line. Whatever you call it, it's damn useful on any system.
Sometimes it's a pain to find though, and it's also hard to remember commands for it.
On the first issue, I've started using Guake Terminal - based off, if I remember correctly, the console in Quake, one key shows a drop-down terminal ready for use, and the same key hides it again with a second touch. It's become so useful it now starts up when I log in.
On the second, there are countless cheat sheets, but I've found it's easier to just experiment with commands you use often, and remember them. Never mind others you won't use, at least not until you do need them.
So now, I've disabled the update manager from doing anything, except once a week interrupting me to tell me I missed an update or two, and now when I log in I run five commands in Guake Terminal:
"sudo apt-get update" to update the package listings
"sudo apt-get upgrade" to handle all upgrades to all packages
"cd svn/openttd" which changes to my source code folder for the OpenTTD game
"svn update" updating the source code to latest nightly, then finally:
"make" which lands me the compiled version.
These five commands mean that after they're all completed, practically everything on my system is up-to-date, and it takes about 5 minutes, including the compiling of OpenTTD.
Handy little package Guake. I recommend any Debian or Ubuntu user to try it, and I do believe somewhere out there is a KDE version too. Knowing their naming habits, probably Kuake. Hmm. Maybe not.
Rock on all.