Apple has introduced new security measures which users can sign up for to help protect their iTunes and App Store accounts and prevent unauthorised users from taking over their accounts.
The two-factor authentication system is similar to Google's new system implemented for Gmail, which uses vertification codes sent in a text message. It is available in the UK, Ireland, the US, Australia and New Zealand, and uses either SMS or the Find My iPhone app to send a numeric code to the user. Logging into the account then requires entry of the user's Apple ID password together with the numeric code.
In a support article introducing the new feature, Apple said: "When you set up two-step verification, you register one or more trusted devices. A trusted device is a device you control that can receive 4-digit verification codes using either Find My iPhone notifications or SMS to verify your identity. Then, any time you sign in to manage your Apple ID at My Apple ID or make an iTunes, App Store, or iBookstore purchase from a new device, you will need to enter both your password and a 4-digit verification code. After you sign in, you can manage your account or make purchases as usual. Without both your password and the verification code, access to your account will be denied."
The new system is not enabled by default. Users need to access their Apple ID account settings to activate the feature within the Password and Security option. It also uses a 14-digit recovery key that a user can employ to gain access in the event of loss or theft of a mobile device or any other reason that the account cannot be accessed. It also renders personal security questions unnecessary.
Two-factor authentication is commonly used in banking and you can find out more about how it works here.