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 (see the section called "Root And Sudo").
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