In a simple example, a lot of YouTube videos have become "viral" in a sense that they spread extremely fast because they are popular. When you have a video with tens of millions of views, you can implement simple marketing techniques either through a service or yourself to reach out to those people who are viewing your video.
A great example would be "Auto Tune The News" on YouTube. These people basically take news videos and edit them into songs and put the extremely popular auto tune effect on the voices of everyone in the video. Autotune basically is a pitch correction software. It is quite funny and since so many people watch their videos on YouTube, they make a lot of their songs available for purchase on iTunes which they make royalties off of.
Hope that helps!