Specific Packages
As you are no doubt aware, with Open Source software you are able to download the source code and compile the application yourself. In many cases this may not be practical and what you would really like is the application available to you already built and packaged up ready to install on your computer. This is where the various Linux Distributions play an important part as a lot of that work is already done for you. Sometimes though your distribution of choice may not have packaged the application at the level you need or even may not have packaged it at all.
All is not lost however. Chances are that someone, somewhere in the world will have struck the same problem and may have compiled and packaged the application themselves and made the package available on the internet.