I have always had to install something, by general I have made it either from the corresponding repository or from the source code (by manually working out the dependences in this last case).
But what happens if we want to install directly a deb package? Typically we are going to employ the dpkg command.
javi@kubuntu:~$ sudo dpkg -i package.deb
If the package to be installed needs some dependence, the dpkg output shows it and we have to manually set it up. But so as to work around this task, gdebi can carry out all these necessary actions: to resolve the dependences and install the final package.
javi@kubuntu:~$ sudo gdebi package.deb
And finally, also say that this tool is available for Debian systems and their derivatives (Ubuntu, Kubuntu, Knoppix, etc.).