Google was one of the first Internet companies to introduce a two-step verification process.
To access a Google service using the two-step verification process, a user has to go through the following two stages:
The first step is to log in using the username and password. This is an application of the knowledge factor.
The implementation of the second step requires a mobile phone or the Google Authenticator application, which is an application of the possession factor.
If the user opts to use a mobile phone, he/she has to register his/her phone number with Google. When one attempts to authenticate with username and password, Google will send via SMS a new, unique code to the phone. Receiving the SMS demonstrates that the user has the phone (or, in the case of GSM like networks the appropriate SIM chip).
If the user opts to use the Google Authenticator (or another supported code generator application), he/she simply opens the application, which generates a new code every 30 seconds. This code is to be entered to complete the log in process. As a backup option in case the registered mobile phone or device running Google Authenticator is lost, stolen, or otherwise unavailable, the user can print a set of static single-use backup codes (also the knowledge factor) and store them in a safe place.