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.
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.
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.