Install .deb

Packages are manually installed via the 
dpkg command (Debian Package Management System). 
dpkg is the backend to commands like 
apt-get and 
aptitude, which in turn are the backend for GUI install apps like the Software Center and Synaptic.
Install a package:
To install a downloaded Debian (Ubuntu) package (.deb): Open Terminal and type
sudo dpkg -i DEB_PACKAGE
To remove a Debian (Ubuntu) package (.deb):
sudo dpkg -r PACKAGE_NAME
To Reconfigure/Repair an installed Debian (Ubuntu) package (.deb):
sudo dpkg-reconfigure packagename