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1. The User Agent - the tool being used by the user to access the application. This could be a web browser, an application such as Panoply, or it could even be a command line tool or script such as curl or wget.
2. The Application requiring user authentication. This could be a simple HTTP based data download service, a wiki, a data search tool, subsetting application, etc.
3. The OAuth2 service - in this case, Earthdata Login.

The process goes something like this:

Step 1

The user agent sends a request to the application for a resource - for example, a web page or file.

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Earthdata Login verifies the credentials, and replies by telling the user agent to now go back to the application. It also provides the user agent with a special 'code or 'ticket' that it must present to the application.

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