A Google Account operates as a single Google login, made up of an email address and password of your choosing. It gives you access to various free Google services, including Google Groups, Google Shopping List, Web History, iGoogle, and more. With a Google Account, you can also sign in to AdWords, AdSense, and Google Checkout. If you've used any of these services before, you already have a Google Account. Your account username is simply the email address you used during the creation process.
If you have a Gmail account, you can use your Gmail username and password to sign in to your Google Account.
You now have the option to add YouTube as a service to your Google Account linking the two. There are a few ways you can do this:
Google Accounts FAQ