Two-factor security is a relatively recent trend in computer security. You can already enable it for many commonly used websites like Gmail, Facebook, and Twitter. It works on the principal that people can achieve better security by requiring both something that you know (typically a password) and something that you have (like a phone app or key card). This makes it extremely hard for someone to gain unauthorized access to your account even if they have one of the pieces (typically your password).

Steam will be enabling access for select users with Android devices starting today. An app will be available for iOS soon.