These files are Debian packages. The package files associated with Ubuntu have the .deb suffix because of Ubuntu’s close relations with the Debian GNU/Linux distribution. You will need administrative privileges to install a .deb file .
To install a .deb file, simply double click on it, and then select Install Package
Alternatively, you can also install a .deb file by opening a terminal and typing:
sudo dpkg -i package_file.deb
To uninstall a .deb file, deselect it in your package manager, or type:
sudo dpkg -r package_name
i would install software in ubuntu through the software centre but since the time 11.04 became obsolete.. the software centre became non responsive ..This should be the first way of installing .. but then again with an improved GUI .. one learns when it is needed