If you have never used the AdWords API, consider reading Get Started. This migration guide goes over the key differences between the AdWords API and the Google Ads API to get you up to speed if you're starting to look into exactly what is changing and what is staying the same.
Here are the main topics that you need to know about in order to understand the Google Ads API from the perspective of a seasoned AdWords API developer.
Here are a few underlying concepts new in the Google Ads API.
The underlying transportation layer for the Google Ads API is now gRPC rather than SOAP, which was used for the AdWords API.
The client customer ID is now part of the URL path when making requests. For example:
The AdWords API allowed the customer ID to be specified in two formats, one with dashes (XXX-XXX-XXXX) and one without. In the Google Ads API, only the version without dashes is accepted, and adding dashes will result in an error. This is true for the endpoint and all resource names that include a customer id.