I first learned about it as a means to block those annoying ads on YouTube, and after trying many others that weren't up to snuff, I can honestly say that in two weeks of using ABP, I have not seen a single ad on YouTube.
But wait, there's MORE! Not only have I not seen a single ad on YouTube, I haven't seen a single ad on any video AT ALL!!! No matter where I go - CNN, Yahoo, Al Jazeera, etc, whether for pleasure or for work, ALL the ads are magically gone, I get sent STRAIGHT to the video I want to watch.
As a bit of news junky, this is a god-send. I used to avoid video news on the web simply to avoid the tedium of the advertisements before. But now, no more. I am free!
I highly recommend checking out AdBlock Plus. You'll be glad you did!