Steam Guard is a mobile authentication platform employed by Steam as a means of extra security applied onto a Steam account.

Steam Guard comes in two forms; email verification and mobile authentication. The former is automatically applied during creation of an account, and the latter is employed by user choice.

Email Verification

Steam Guard automatically employs email verification upon the creation of a Steam account. When logging into Steam from a device that it does not recognize, the user will be prompted to check their email for a code that is required to login to their account.

The Mobile Authenticator as viewed from the Steam application.

Mobile Authenticator

Users may also opt to employ Steam Guard Mobile Authenticator for extra security and added benefits. The user must first install the Steam application on their mobile device and enable Mobile Authenticator from there.

Users without Mobile Authenticator enabled will have Market listings and trade offers placed on hold after listing or accepting them. These holds may bypassed upon enabling the Authenticator.