Synaptic is a graphical interface for Aptitude, the package manager in Ubuntu/Linux. It provides a listing of available software packages that are available to download and install from the repositories listed in your Software Sources. Synaptic is less intimidating to use than apt-get in a Terminal interface for users not used to dealing with the command line.
Preceding commands in Terminal/command line with sudo imparts a restricted root-user ability for performing commands that are not available for standard user accounts.
When in doubt about any basic command in Terminal, you can always man [base command] to access the command manual pages. man sudo will show the manual for sudo, for example. There is also extensive documentation on the Ubuntu website for most commands.
As for installing Opera, it is not maintained in the standard Ubuntu repositories. You have two options:
1) add the Opera repository to your Software Sources as mentioned in the other answer and following the directions in the provided link; or
2) download and install Opera manually.
To install Opera manually, go to the Opera website (www.opera.com) and download it from there. Presuming that you have your downloads set to go to your Downloads Folder, you can run the following in Terminal:
cd /home/your_account/Downloads ;ls -l
Replace "your_account" with your short name; the "ls -l" will list the contents of the Downloads folder. Locate the Opera package file - it should be something like 'Opera_12.15.1748_amd64.deb'.
sudo dpkg -i Opera_12.15.1748_amd64.deb and enter your password when prompted. This will run the Opera installer.
In order to have Opera updated automatically, you should still add the Opera repository to your list of software sources.
Try this:
Open a terminal,
Press Ctrl+Alt+T
Run it:
exec sudo -i
nano /etc/apt/sources.list.d/opera-stable.list
In the file that opens, paste this text:
# This file makes sure that Opera Browser is kept up-to-date
# as part of regular system upgrades
deb https://deb.opera.com/opera-stable/ stable non-free #Opera Browser (final releases)
Ctrl+O
Save file.
Ctrl+X
Close nano.
Continue running in the terminal:
wget -O - http://deb.opera.com/archive.key | apt-key add -
apt-get update
apt-get install opera-stable
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Note: If you are using a version of Ubuntu that doesn't include the software center by default, install it first with the command
sudo apt-get install software-center