API stands for "application programming interface." This Wikipedia article covers APIs in more detail, but it can be helpful to think of the API as a way for different apps to talk to one another. For many users, the main interaction with the API will be through API access key, a way for other apps to access your account without you giving out your password.
The QuestionPro API is organized around REST. We use built-in HTTP features, like HTTP authentication and HTTP verbs, to allow you to interact securely with our API from a client-side web application. JSON will be returned in all responses from the API, including errors.