Google APIs are the web APIs through which programs work with Google products and with the data that users keep in them, for example their Gmail mail. The Google APIs Explorer, on Google's developer site, lets a developer try each method from the reference pages without writing code.
To reach a user's private data, an application uses OAuth 2.0, a standard for delegated authorization. The developer gets client credentials, such as a client ID and a client secret, from the Google API Console. The application asks the Google Authorization Server for an access token; the user signs in and approves the requested scopes, which limit what the token allows. The application sends the token with each API call and refreshes it when it expires.
Use falls under the Google APIs Terms of Service and the Google API Services User Data Policy. The policy requires a published privacy policy and only the permissions an app needs. Apps that use sensitive or restricted scopes must also follow Limited Use rules and, depending on the API and the number of users, pass an annual security assessment.