Because our online accounts have become so important, both Google/Gmail and Facebook now offer two-step verification for enhanced security. The basic concept in two-step security is that in addition to your username and password, whenever you login into a new machine/browser you will verify your identity with a single use verification code that is sent to your phone. For details on the Google system, read this support article. Facebook just started rolling out their two-factor authentication this week. Learn more with the Lifehacker article.