If you use a Debian-based Linux distribution then you probably use Synaptic. And if you use Synaptic enough you may have come across an application that it can not find. More here
Why do applications like Synaptic exist? Simple. Unlike Microsoft Windows where you have to search site after site for applications, or must purchase the boxed goods, Linux likes to have everything in one place. With tools like synaptic Add/Remove Software actually helps you by keeping a vast software repository in one, easy to use, location. That location is helped with GUI front-ends like Synaptic.