To install a package:
apt-get install [package name]
Or:
apt-get install [package name]
To uninstall a package:
apt-get remove [package name]
Or to uninstall a package and remove config files as well:
apt-get remove --purge [package name]
To install a package that you have downloaded from internet ie skype:
dpkg -i [package name]x.x.x.deb
To remove it:
dpkg -e [package name]x.x.x
To search for packages:
apt-cache search [search pattern]
To reconfigure a package, ie xserver-xfree86:
dpkg-reconfigure [package name]
To search for an installed package:
dpkg -l | grep [search pattern]