- The Access API
- The Plug-In API
- The Runtime Environment
The Access API is where a "traditional web" API is provided (probably a web service API exposed as SOAP or a RESTful interface.) Your code runs outside the platform (think using the Flickr API on your blog.)
The Plug-In API, long common in desktop apps, is starting to appear on the web – e.g. Facebook apps. Your code still runs outside the platform, but it appears within the platform.
The Runtime Environment provides an environment in which your code is hosted and runs. Something like Popfly is an example of the third type – but, interestingly, allows you to consume and combine the first type. This third type of platform lowers the barriers to programming – by providing a hosting and runtime environment and, in the case of Popfly at least, loads of examples – and sharing the results of your programming adventures.