Dialogflow ES is a natural language platform owned by Google. (Formerly known as just Dialogflow or as api.ai.) A fulfillment in Dialogflow ES allows an intent to perform operations on input parameters and return an output. Use this tag when issues arise with building fulfillments. For questions about the Dialogflow ES API or UI, use the dialogflow-es tag. For the similarly named, but incompatible, Dialogflow CX, use the dialogflow-cx tag.
Dialogflow ES is a Natural Language Understanding system from Google and was formerly known as "Dialogflow" or "api.ai". It provides NLU processing for relatively simple conversations.
One of the components of Dialogflow ES allows developers to create code in a webhook fulfillment to apply business logic after an Intent has been matched to process the input from the user and apply any business logic to determine a response.
The webhook is called after the user input has been processed and narrowed into a defined Intent. If you are interested in writing code that sends user input to Dialogflow via an API, manipulates the conversational model, or otherwise handles the response after it has been processed by a webhook, you should use the dialogflow-es tag.
For the more complex, and completely incompatible, NLU system from Google called Dialogflow CX, which also includes a visual flow editor as well as an API and fulfillment system, see the dialogflow-cx tag.