How to install .deb file in ubuntu using terminal?

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.
Something along the lines of:

dpkg --> apt-get, aptitude --> Synaptic, Software Center

The easiest ways to install a package would be, first, the GUI apps (Synaptic, Software Center, etc..), followed by the terminal commands apt-get and aptitude that add a very nice user friendly approach to the backend dpkg.

Since dpkg is the base, you can use it to install packaged directly from the command line.

DPKG commands

There are two actions, they are dpkg-query and dpkg-deb.

Install a package

# sudo dpkg -i {package_name}    
# sudo dpkg -i skype-ubuntu-precise_4.2.0.11-1_i386.deb

Remove a package

# sudo dpkg -r {package_name}
# sudo dpkg -r vlc

Remove a package and its configuration files

# sudo dpkg -P {package_name}
# sudo dpkg -P vlc

List all installed packages.

You can pipe the command to less (a pager) so you can more easily scroll the content:

# dpkg -l | less

Check if the package is installed or not

# dpkg -l {package_name}
# dpkg -l vlc

Check if the package is installed or not, and if it is, launch it:

# dpkg -l | vlc

See whether a package is installed or not

And this will show the location where the package will be installed. Here -S (capital S) to search whether the package was installed or not.

# sudo dpkg -S {package_name}
# sudo dpkg -S skype

Install a *.deb package from a specified location

Here -R is recursive. (Recursively handle all regular files matching the pattern *.deb found at specified directories and all of its subdirectories).

# sudo dpkg -R --install {package_location}
# sudo dpkg -R --install /home/sysadmin/soft

Show package details

Here -p (lowercase p) will show the package info:

# dpkg -p {package_name}
# dpkg -p apache2

View the content of a package

Use -c (lowercase c) to show the content:

# sudo dpkg -c {package_name}
# sudo dpkg -c skype-ubuntu-precise_4.2.0.11-1_i386.deb

Extract the *.deb package file

Use -x (lowercase x) to extract:

# dpkg -x {package_name} {location_were_to_extract}
# dpkg -x libqt4-phonon_4.6.3-4+squeeze1_i386.deb /home/sysadmin/

Extract and display the filenames contained in a package

Use -X (uppercase X) to display the content with extraction.

# dpkg -X {package_name} {location_were_to_extract}
# dpkg -X libqt4-phonon_4.6.3-4+squeeze1_i386.deb /home/sysadmin/

Display information about a package

Here -I stands for information:

# dpkg -I {package_name}
# dpkg -I libqt4-phonon_4.6.3-4+squeeze1_i386.deb

Reconfigure an already installed package

sudo dpkg-reconfigure packagename

dpkg-reconfigure reconfigures packages after they already have been installed. Pass it the name(s) of a package or packages to reconfigure. It will ask configuration questions, much like when the package was first installed.

# dpkg-reconfigure postfix

This will reconfigure postfix the same way as when you installed it for the first time.
Need to know more about dpkg commands? Have a look at the manual page:

# man dpkg

A handy tip when installing a program like Libreoffice which has multiple .deb files in a folder is to use.

sudo dpkg -i *.deb

Create your own script installer debInstaller as the following:

#!/bin/bash
dpkg -i $1
apt-get --yes --fix-broken install

Make the script executable with

chmod +x debInstaller

Then move it to some dirs in your PATH or add the current directory to your PATH.
I'm going to move it to /usr/bin

sudo cp debInstaller /usr/bin

Now you can install any .deb package using the command:

sudo debInstaller some-package.deb