The Google Ads API can be called either using gRPC or REST. Both interface types expose a resource-oriented design shared with other Google Cloud APIs. Consult the Google API Design Guide for more context on the architecture of the Google Ads API.
This guide presents the design and working details specific to the API's REST implementation, and explains what you need to know to call the REST interface directly, without the use of a Google-supported client library. The information from this guide would be useful if you're writing your own custom code to directly access the REST API or are using a third-party HTTP client library.
We strongly recommend using Google-supported client libraries wherever you can. Although most of our client libraries are implemented using gRPC, the Perl library is implemented on top of the REST interface. It may serve as a useful reference implementation for many of the concepts discussed in this guide.