What Cookies Store
Cookies are only as dangerous or invasive as you, the Web page viewer, allow them to be. If you fill out a form with your name, address, and phone number, that information can be stored in an HTTP cookie on your machine. The important part to remember is that a cookie can't know more information than you provide to it. If you don't want a Web site to have your phone number, then don't fill in a form that asks for it.
Cookies are often used to gather demographics about a computer user. The company doesn't know who you are or where you live or anything like that, but every time you accept a cookie from them, you tell them a little bit about where your computer has surfed that day. Specifically, the page that you are currently on, and the page that you came from.
If you frequent running Web pages and natural food store pages, in theory, companies can use that information to target banner ads to your computer. In practice, however, cookies are more often used to track the habits of thousands of people in aggregate.
The Personal Information Isn't Very Personal
Cookies can only use information that is provided to them by either the Web browser itself, or by the customer filling out a form and providing the information. If you have information you would like to keep personal, you should not type it into a Web form. If you type it in, it can be stored in a cookie. But if you're willing to tell a Web page personal information, how personal is it, really?
About.com uses a lot of cookies. Most of these are to track advertisements. The idea is, if you've seen an ad for a new computer 3 times and haven't clicked on it, chances are a 4th time won't do the trick. Using a cookie can track how many times you've seen a particular ad and gauge it's success rate.
What Else Can Be Stored in Cookies?