But I know enough to know that I don't want to find out. It's generally a matter of following the same common-sense rules that you follow in real life. Just as you would never give your house keys to people on the street, never tell anyone your password to someone who asks for it in a chat room or instant message. Same thing for your Social Security and drivers license numbers. Someone with malicious intent can use that information to fabricate a new identity for himself - and make a lot of trouble for you.
Some Web sites sell their e-mail address lists to junk mailers.
Look for sites that promise to keep your address to themselves. Some consumer groups warn you should never use credit cards online, but that's old advice. It's OK if you're buying something from a Web site that conducts secure transactions. A site does that by encrypting your credit card information, or turning it into a secret code, when you order merchandise, making it not worth a hacker's time to intercept your number. Obviously, sites with secure transaction systems are safer than sites without similar setups.