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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