We encourage our users to make videos using Valve game content, such as playthrough or instruction videos or SFM movies. We are fine with publishing these videos to your website or YouTube or similar video sharing services. We're not fine with taking assets from our games (e.g. voice, music, items) and distributing those separately.
Use of our content in videos must be non-commercial. By that we mean you can't charge users to view or access your videos. You also can't sell or license your videos to others for a payment of any kind.
You are free to monetize your videos via the YouTube partner program and similar programs on other video sharing sites. Please don't ask us to write YouTube and tell them its fine with us to post a particular video using Valve content. It's not possible to respond to each such request. Point them to this page.
Of course this policy applies only to Valve content. If you include someone else's content in your video, such as music, you will have to get permission from the owner.