The AdWords API is a collection of web services that you can use to build applications that manage AdWords accounts and their associated campaign data. While the AdWords API is based on SOAP 1.1, high-level client libraries are provided to help you develop applications more quickly.
This guide assumes you are familiar with AdWords fundamentals, SOAP concepts, and whichever programming language you will use to interact with the API.
Making Your First Request shows you how to add a new campaign step-by-step in all the languages supported by the client libraries.
For usage examples and details about the services that make up the AdWords API, browse through Services & Use Cases.
After you're comfortable with the material in Basic Concepts, you can proceed to explore the other topics (e.g. authentication or reporting) in the Developer's Guide. For hints on debugging and best practices, look through the pages under "Troubleshooting and Performance".