What is a Developer Token?
A Developer Token is a unique combination of letters, numbers and characters that identifies your AdWords API activity. It is your key to talking to the AdWords server and your clients’ AdWords accounts. In order for us to identify your activity, you should include the Developer Token in the header of all your API requests.
To protect yourself from fraud, do not share your Developer Token with others. You can find your Developer Token through your AdWords API Center – accessible from your My Client Center’s My Account tab.
Please note that developers need both an approved Developer Token and an approved Application Token (associated with each API application) to access the API. Read this FAQ to learn more.
Source : Google AdWords API Support
