Ubuntu, trying hard not to look like the GNU/Linux of old, gives it's users a single graphical pipeline to install software over the Internet, called the Ubuntu Software Centre. But under the candy coating of the Unity graphical interface, all the old ways of installing software still work, and many would argue they still work much better than the user friendly, lumbering Software Centre  -- I say it's good to have choices. One benefit of understanding the various methods, is that it gives you the freedom to install most any Linux software, not just the wares in the Software Centre.

Ubuntu  software repositories
Personal Package Archives (PPAs)
Debian packages (.deb)
Python packages (.py) and portable Linux applications
Compiling from source
Non-Debian Linux packages
Your friend, the Command Line

Ubuntu software repositories ▲

Software is a fundamental part of what sets Ubuntu apart from other Linux distributions -- which programs are installed by default, which are included in the software repositories. These repositories are where the Ubuntu Software Centre, and the Synaptic Package Manager get their lists of installable software; it's the first place the underlying package management functions, apt-get, look.There are thousands of programs, with an eye towards covering all the bases several times over. the repositories are also the safest way to play, compatibility-wise. One compelling reason to look beyond Ubuntu's choices is the fact that a lot of the software in the official repositories is old.

The settings relating to software repositories can be viewed, and edited by launching the Ubuntu Software Centre, and, from the main menu, choosing: Edit > Software Sources... If you touch nothing else under the Ubuntu Software tab, you may want to change the Download from: location. This is the server which your system will download repository software from. Sometimes the default choice isn't the best for your location (you can always change it back). I'd leave everything else, with one exception, until you understand what you are doing. The one exception is the updates tab. I personally don't need Ubuntu to check for updates every day.

Personal Package Archives (PPAs) ▲

I'm running Ubuntu 12.04. When I install GIMP (Golly It Mimics Photoshop), the software repository will deliver up the older GIMP 2.6. If I want the latest version, 2.8.2, I can get it via the GIMP developer's Personal Package Archive. This is performed in the Terminal Emulator with three simple commands (from here):

sudo add-apt-repository ppa:otto-kesselgulasch/gimp
sudo apt-get update
sudo apt-get install gimp

People want GIMP 2.8 because it is a very major update, with lots of new, and improved features, such as the Single Window Mode. I also install Qbittorrent 3.0 via this PPA.

Debian packages (.deb) ▲

Just like Soylent Green is people, Ubuntu is Debian Linux, with more icing. I believe all the packages in the official Ubuntu software repositories are Debian packages. And there are loads of Linux programs not available in the Ubuntu repositories, but available in the wild as Debian "deb" packages. You are supposed to be able to  install local deb files with the Ubuntu Software Centre, except the Software Centre, in my experience, rarely can do it, rarely even can "see" the deb file. One of the first programs I install in Ubuntu, through the USC, ironically, is the graphical GDebi Package Installer -- it easily installs downloaded deb packages and resolves and installs dependencies. Once it is installed, you can right-click a deb file and choose to open it with GDebi. That's that.

Python packages (.py) and portable Linux applications ▲

I had a surprise, shortly after I first installed Ubuntu 12.04. I began to Gnome-2-ify Ubuntu. I found a nice Classic Menu Indicator, installed via PPA, and then a Places indicator, called indicator-places; this you downloaded as a zip file. The instructions were: "Extract the files / Run indicator-places.py." It was like a Macintosh application; It was self-contained; it was portable -- I was pretty delirious about it. The few other python applications I've used have been like-wise portable, like Scrambled Egg, an encryption program.

There are a number of so-called portable application projects for Linux. Most are languishing. Like a lot of Linux efforts, the big enthusiasm was back in 2009. Some examples from my records:

TreePad Lite 2.2.1 for Linux -- a hierarchical tree-view note program. self-contained, and portable; written in an old software development system called Kylix. It ran, but wonkily, I recall. The Treepad folk also rewrote the Linux version in Lazarus for version 4.3. Available on the same page, and, also portable, I recall.

QBittorrent 2.4.9 from this Sourceforge page of 210 portable Linux applications, dating from 2010 to 2013. I was able to install this Qbit in an install of Precise Puppy.

Compiling from source ▲

My only advice is find another way to get it into your Ubuntu box. I once, successfully installed aircrack-ng from source -- it was a real, real pain. On another computer, a year later, I wisely hunted down a deb file, and installed it in two minutes. Turns out I have no real interest in that sort of program.

Non-Debian Linux packages ▲

Alien is CLI program which can convert Redhat (RPM), Stampede, and Slackware software packages to Ubuntu-compatable Debian packages, and, apparently vice versa. It's available in the Ubuntu Software Centre. I've only used it once, to successfully install from a RPM package. There is a GUI front end, called Package Converter, which I've installed, via this PPA, but not yet used.

Meet apt-get, the brains of the operation ▲

If Ubuntu isn't a deliberate scheme by Linux developers to convert unsuspecting computer users over to the Terminal, and the Command Line Interface, then it can certainly act that way -- at least it has for me.

The Terminal Interface looks like a window on the past, before elegant Graphic User Interfaces (GUIs), when the Command Line Interface (CLI) was the only Interface. GUIs were thunderously popular, opening the possibilities of computers to millions of people. But, cried a few,, thunder was good, thunder was impressive; but it was the lightning-fast CLI, which got the work done. ►

The Ubuntu Software Centre is actually a GUI front end for a set of CLI programs collectively called apt-get, which do the actual work of managing software. The Software Centre may be a bit of a lacklustre jewel in Ubuntu's GUI crown, but apt-get is a true CLI crown jewel.

The apt-get package management system was developed by Debian. Ubuntu is a version or "flavour" of Debian, and much of what tastes good in Ubuntu is Debian. Apt-get is a great way to dip your toes in the Command Line Interface. Apt-get is the CLI, at it's best; simple, short, easy-to remember commands, and it works fast. One aspect of apt-get is it's use of software repositories: Each version of Ubuntu has it's own software repositories. So, no need to remember version numbers; each repository will include the program GIMP: In Ubuntu 12.10, it's GIMP 2.8, in Ubuntu 12.04, it's GIMP 2.6. In the Terminal you just type:

sudo apt-get install gimp

► In Ubuntu, sudo temporarily  allows you to do what you want.
► Here we tell Ubuntu to launch the apt-get set of programs.
► The apt-get function we want to use. There are many, including "remove."
► What we want to install.

This is pretty close to a real English sentence: "Let me use apt-get to install gimp." Like a sentence, word order, or syntax is important. In the CLI, a space is like a period -- in English, periods separate thoughts; in the CLI spaces separate commands. apt-get need the hyphen to be seen as one command. For the most part, everything is typed in lower case.

The above use of apt-get is the simplest example; great if you know the name of the program you want to install or remove. Apt-get doesn’t, so far, as I know allow you to search the Ubuntu repositories, though it will be "smart" enough to suggest the package you may want, when you type wrong, or try to install an obsolete program.

The function "apt-cache" does allow you to search available software packages from the Command Line. Here's better instructions than I could give. ►

The now deprecated "GNU," in GNU/Linux, refers to a most curious beast, who is named after itself, and whose creation began with the hind quarters and worked it's way forward. The GNU was admittedly wondrous in it's parts, but it's creator, fittingly named Richard Stallman, never quite finished it's brain. Coincidentally, a Finnish-ed brain, created by Linus Torvalds, was found just lying around on the Internet, and lo-and-behold, it fit! Thus was born the GNU/Linux. Like the old African adage, "It takes an Internet to raise an Operating System," the wobbly beast was raised free-range by the Opensource community. Richard Stallman, was honoured for coming up with the idea, masterminding 90 percent of the work, and being just a tad too uncompromising, by being locked away in the attic like a crazy uncle. The Opensource movement apparently also won a restraining order against the word GNU, forbidding it from appearing within 100 metres of the word Linux. The Wizard of Os, Linus Torvalds, who gave the GNU it's brain (but has been unable, or unwilling to give Mark Shuttlesworth much in the way of courage, where it comes to Microsoft, his so-called Bug #1, is continually honoured as the father of Linux, and was awarded a prestigious Duchy in largely-uninhabited Google+land, from which he famously ranted against Gnome 3 (and which he, just as famously, has switched back to).