You can "monetize" your videos on YouTube. You have to set up an account on Google AdSense to receive payment into, then you're good to go.
When you monetize your videos, viewers will start to see ads embedded in them (you have options as to how the ads are inserted). It's the ads that generate the revenue (pennies per 1000 views).
Got it. Like the British chick doing the POOPouri commercial on the toilet.