Be sure to wash your hands after surfing
Think about how many times a day you click to watch a YouTube video, no matter which site it's on. It might be Obama's weekly chat at change.gov, or even Consumer Watchdog's YouTube video on Chrome's privacy problems. Before you even click to watch the video, you collected several YouTube cookies. And after you click to watch, about ten seconds into the video, Google reads your universal google.com cookie. This is the one with the globally-unique ID. It used expire in 2038, but now it pretends to expire in two years. However, every time you visit any Google site, it gets pushed two years ahead, which means it expires when your hard disk is replaced.
If you don't already have a Google cookie, you get a new one with a new ID. If you have one already, it reads the old cookie. Put your PC on a packet scanner and click on a YouTube video. The GET request to google.com, which apparently is done from the embedded Flash code from YouTube, includes the site you are on, as well as the video you are watching.
This information is available to the U.S. government without a court order. It's called a "National Security Letter" and when Google gets one, it comes with a gag order. How many other governments around the world have similar laws?
Delete your Google cookies and your YouTube cookies when you exit your browser. It's common-sense hygiene - the equivalent of washing your hands after you visit a dirty bathroom at a gas station.