Apt-Get and Synaptic

*Regardless of what distro you are using the key is in the repositories. If you are using a distro that uses apt-get and synaptic, you can find a list of the sites your distro connects to for getting its packages by checking your /etc/apt/sources.list. Please ask someone who uses the distro you have about what the best repositories are before diving in.
Apt-get Commands
apt-get update - updates the sources files - do this nearly every time you run apt.

apt-get upgrade - upgrades all the newer packages recognized by synchronizing with the servers.

apt-get dist-upgrade upgrades to all the latest packages and is usually associated with changing to a newer repository - don't use this command unless you know what you are doing, just stick to apt-get upgrade if you want to be safer.

apt-get install [name of package/app]

apt-get remove [name of package]

apt-cache search [name of package] - searches for any package containing the specified name.

apt-get clean - removes the downloaded package files - saves space on your hard drive.
Synaptic

Synaptic is the graphical version of apt-get

Here are some screenshots of Synaptic in action:
Searching for: java, mplayer, gcc, and the w32codec for playing quicktime trailers on the apple site.

*This is based on Debian's apt-get package manager but has been ported to other distros like Red Hat, Feather, Knoppix, Damn Small Linux and SuSE among others.