Out of the 12.6 million Americans who were victims of identity theft in 2012, as many as 1.5 million knew the person who did it. It's called familiar fraud, and it usually involves your social security number and banking information. But it's not only your family and friends who can't be trusted; right now you probably have a phishing scam waiting for you in your inbox.
Phishing scams look something like this: Someone poses as a legitimate business, usually through e-mail or on the phone. Scammers could pose as a financial institution (a common ruse involves feigning to be the fraud department at your credit card company) or even a government agency such as the IRS -- and "phish" for information such as your social security number, your PIN number and any other personal identifying information.
Be on the lookout for suspicious e-mails, as well as any links and attachments, which could lead you to a malicious site spoofing the legit one or install spyware on your computer. Be suspicious of anyone asking for your personal information over the phone -- if they're the real deal, they won't ask for your password or PIN.
Also, be aware of how much information you're sharing about yourself even when no one is phishing for it. Always browse securely, never publicly post any personal identifying information, and wipe clean any computer or mobile device before you get rid of it.