You can call HTTP functions yourself from a browser (if GET is enabled), from your code (using an HTTP client), or using another program that supports HTTP requests. Another really cool feature that's at the core of functions is that they can be triggered by various events, thereby enabling an event-driven programming architecture. For example, a function can be triggered when a message is put on a Service Bus queue, when a blob is changed or added to a storage account, or when a document changes in Cosmos DB. There are various other triggers that I personally haven't needed yet, but there are triggers for Event Grid, Azure Active Directory, Outlook, and more. You'll see them for yourself when you create a new function.