how to install in terminal

suppose u want to install mp3, flash player through teriminal
type the command after $
$ sudo apt-get install ubuntu-restricted-extras

if u r in some loacation like home$
in home u have a file opera*.deb. then u can unzip and install like this
$ sudo dpkg -i opera*.deb
$ sudo aptitude install opera

To install software via the command line in a terminal, you can type (without quotes) "sudo apt-get install packagename", where package name is the name of the software you want to install, and press enter. You will then be prompted to enter your password (which for security reasons will appear blank as you enter it - don't worry, it's still working!) and press enter again and the installation process will begin!

For example (this example is for firefox):

You open a terminal, and get a prompt (~$)

~$ sudo apt-get install firefox
[sudo] password for foobar: enterpasswordhere (foobar is just a random name I made up, obviously your name would be there and you would enter your password)

and then it installs!

Thanks Sam, that solved my question.