- Things that require you to make significant or potentially dangerous changes require root or administrator level permission, Ubuntu prefers you don't simply run around as the root user, so sudo gives a command permission to do things for the root user. Putting it in a command basically tells the computer, "Yes, it's ok to do this".
- Accesses the software repositories, a giant collection of various programs for Ubuntu users
- Searches through a stored list of programs in the repositories for your keyword. After the keyword, add to narrow down your results even more.
apt-cache search "keyword"
- Searches the repositories for the package specified and installs it, since it makes changes to your hard drive, sudo must be used.
sudo apt-get install "package"
- Both of these are used on a fairly regular basis, updates your repository information, any security updates, program patches, etc. Basic upkeep of your current software.
sudo apt-get update/upgrade
- Removes a program from your system, leaving configuration files to be used if it is reinstalled.
sudo apt-get remove "program"
- Totally removes the chosen program from your system along with configuration files. Best thing if you know you won't be using that program again in the future.
sudo apt-get purge "program"
I think that's really it for the basics of installing/finding/removing things with the terminal. I personally hate the software center, so this is how I install everything.