The /etc/apt/sources.list contains list of registered repositories. This file is filled in with those links during system installation. However, you can freely add/remove links from it. Each line represents a repository (basically a mirror/server) from which you would download specified packages when installing them using apt-get.
Synaptic – Since apt-get let’s you get and install packages from the terminal, Synaptic Package Manager uses GUI to display all available packages that you can download.
Aptitude – Its basically same as apt, but a bit different. Aptitude works on terminal, and prints queries in a tui-like (terminal-user-interface) look. If you ask why aptitude is used for, well, I have no idea, but it is.
Install/Download packages:
Prevent already installed package from upgrading
# apt-get install openssh-client --no-upgrade
Do not install new packages, just upgrade existing package
# apt-get install openssh-client --only-upgrade
Install package with specific version (=)
# apt-get install openssh-client=VERSIONNUMBER
Query packages
Search the local repo in the system, and install dependencies for the package
# apt-get build-dep openssh
Download source code, use –download-only source
# apt-get --download-only source openssh
Download and unpack source code of a package
# apt-get source openssh
Download, unpack, and compile source code
# apt-get --compile source openssh
Just download the package
# apt-get download openssh
Check for broken dependencies
# apt-get check
Update packages
Fetch packages from repositories, update them to newer version
# apt-get update
Upgrade all currently installed packages on the system
# apt-get upgrade
Remove packages
Delete all .deb files from /var/cache/apt/archives to free space
# apt-get autoclean
Remove package, leave its configuration files
# apt-get remove openssh
Completely remove package, together with configuration files
# apt-get purge openssh
Clean up disk by removing downloaded .deb packages from local repository
# apt-get clean
Remove installed package and its dependencies
# apt-get autoremove openssh