Apple is beefing up the security of its Apple ID by adding two-factor authentication to the account login process. Customers concerned about unauthorized access to their Apple ID can login to their account at Apple's My Apple ID webpage and turn on the feature as described below:
Go to My Apple ID (appleid.apple.com)
Click the "Manage your Apple ID" button to login to your Apple ID
Enter your Apple ID and password and click "Sign In"
Select "Password and Security" in the left-hand column
Type in the answers to your account security questions if you are prompted to answer them.
You will see Two-Step Verification at the top of the page. Click on "Get Started" and follow the on-screen instructions.
If you have two-factor verification enabled, you will be required to enter both your password and a 4-digit code to verify your identity. According to Apple's support page, you will need this information whenever you sign in to My Apple ID to manage your account, make an iTunes / App Store / iBookstore purchase from a new device or get Apple ID-related support from Apple. You can read more about the security feature on Apple's support website, and check out Glenn Fleishman's thorough pros and cons rundown on TidBITS.