Because Google bought the rights for youtube they are connected, like sisters.
The great think about having a google account, is when you go out on the internet and post comments or blog, you will get a picture called a gravatar and it will identify you. You can use a personal pic (i do) or a comical one of your choice.
basically google connects your youtube account to your email account ( i use hotmail) but I sign in with google.
Everytime you do something on google, it puts a cookie on your pc, and it tracks what sites you go to.
They know how old you are by your registry for various sites.
So let's say your 14 years old, and you like music, so it will keep track of what kind of "youtube"videos you like to watch.
if you go on a youtube channel and hit "insight" this explains to the channel person that owns it, what age group likes their videos.
Most musicians want to target a certain age group, so they can expand their sales into toys, clothing and games. If they know what AGE is looking at their things, they know how to SELL that information to a company, that creates marketing ideas, so they all make money off you visiting various websites.
if you like games, they will make more games like the one's that 14 year old's like then target you with adds on your email account and sites that you visit.
that's how that works, hope I explained it okay for ya.
http://www.twitter.com/theideagirl
any more questions you can find me under idea girl consulting on blog catalog, wordpress and the idea girl too, linda randall on facebook.